Heterocyclic compound, light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

Provided are a heterocyclic compound which emits blue light and is represented by General Formula (G1) below, and a light-emitting element, a light-emitting device, an electronic device and a lighting device which are formed using the heterocyclic compound represented by General Formula (G1) below. The use of the heterocyclic compound represented by General Formula (G1) makes it possible to provide a light-emitting element which has high emission efficiency, and also a light-emitting device, an electronic device and a lighting device which have reduced power consumption.

This application is a continuation of U.S. application Ser. No.15/789,210, filed on Oct. 20, 2017 which is a continuation of U.S.application Ser. No. 14/947,827, filed on Nov. 20, 2015 (now U.S. Pat.No. 9,796,736 issued Oct. 24, 2017) which is a continuation of U.S.application Ser. No. 14/301,957, filed on Jun. 11, 2014 (now U.S. Pat.No. 9,196,836 issued Nov. 24, 2015) which is a continuation of U.S.application Ser. No. 12/942,214, filed on Nov. 9, 2010 (now U.S. Pat.No. 8,771,840 issued Jul. 8, 2014), which are all incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a heterocyclic compound. Also, thepresent invention relates to a light-emitting element, a light-emittingdevice, an electronic device, and a lighting device using theheterocyclic compound.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Inthe basic structure of such a light-emitting element, a layer whichcontains a light-emitting substance is interposed between a pair ofelectrodes. By voltage application to this element, light emission fromthe light-emitting substance can be obtained.

Since such light-emitting elements are self-luminous elements, they haveadvantages over liquid crystal displays in having high pixel visibilityand eliminating the need for a backlight, for example, thereby beingconsidered as suitable for flat panel display elements. Light-emittingelements are also highly advantageous in that they can be thin andlightweight. Furthermore, very high speed response is one of thefeatures of such elements.

Furthermore, since such light-emitting elements can be formed in a filmform, they make it easy to provide planar light emission, therebyachieving large-area elements utilizing planar light emission. This isdifficult to obtain with point light sources typified by incandescentlamps and LEDs or linear light sources typified by fluorescent lamps.Thus, light-emitting elements have great potential as surface lightsources applicable to lightings and the like.

A light-emitting element utilizing EL is driven by injection ofelectrons from a cathode and holes from an anode into a layer containinga light-emitting substance which is interposed between a pair ofelectrodes. The electrons injected from the cathode and the holesinjected from the anode recombine in the layer containing thelight-emitting substance to form molecular excitons. The molecularexcitons release energy in returning to a ground state. In the casewhere the energy is released as light having a wavelength correspondingto that of visible light, light emission can be seen. Excited states oforganic compounds can be a singlet state and a triplet state, and lightemission can occur either of the excited state.

The emission wavelength of a light-emitting element is determined by thedifference of energy between the ground state and the excited state,that is, an energy gap. Therefore, by appropriate selection ormodification of a structure of the molecule that contributes to lightemission, any color of light can be obtained. When a light-emittingdevice is fabricated using light-emitting elements capable of emittinglight of red, blue, and green, which are the three primary colors oflight, the light-emitting device can perform full color display.

Manufacture of high performance full-color light-emitting devices needsred, blue, and green light-emitting elements which are excellent inlifetime, emission efficiency, and the like. The recent development ofmaterials has achieved good characteristics of red and greenlight-emitting elements. However, as for blue light-emitting elements,sufficient characteristics have not been obtained. For example, PatentDocuments 1 and 2 reported a light-emitting element having relativelyhigh emission efficiency. However, in order to realize high performancefull-color displays, further higher emission efficiency have beenrequired.

REFERENCES Patent Documents

[Patent Document 1] International Publication WO 2008/143229 Pamphlet

[Patent Document 2] International Publication WO 2005/113531 Pamphlet

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel heterocyclic compound that exhibits blue light emission. Anotherobject of one embodiment of the present invention is to provide alight-emitting element having high emission efficiency. Still anotherobject of an embodiment of the present invention is to provide alight-emitting device, an electronic device, and a lighting device ineach of which power consumption is reduced by use of the abovelight-emitting element.

An embodiment of the present invention is a heterocyclic compound havinga structure represented by General Formula (G1) below.

In General Formula (G1), Ar¹ and Ar² independently represent asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, R¹to R¹¹ independently represent any of hydrogen, an alkyl group having 1to 4 carbon atoms, and a substituted or unsubstituted aryl group having6 to 13 carbon atoms, and A represents a substituent represented byGeneral Formula (S1) or (S2). In General Formulae (S1) and (S2), Xrepresents oxygen or sulfur, and R¹² to R¹⁸ independently represent anyof hydrogen, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.

Another embodiment of the present invention is a heterocyclic compoundhaving a structure represented by General Formula (G2-1) below.

In General Formula (G2-1), Ar¹ and Ar² independently represent asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, Xrepresents oxygen or sulfur, and R¹ to R¹⁸ independently represent anyof hydrogen, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.

Yet another embodiment of the present invention is a heterocycliccompound having a structure represented by General Formula (G3) below.

In General Formula (G3), Ar¹ and Ar² independently represent asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, Xrepresents oxygen or sulfur, and R¹ to R⁷ and R¹² to R¹⁸ independentlyrepresent any of hydrogen, an alkyl group having 1 to 4 carbon atoms,and a substituted or unsubstituted aryl group having 6 to 13 carbonatoms.

Another embodiment of the present invention is a heterocyclic compoundhaving a structure represented by General Formula (G4) below.

In General Formula (G4), Ar¹ and Ar² independently represent asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, andX represents oxygen or sulfur.

As the heterocyclic compound of one embodiment of the present invention,there are a dibenzofuran derivative and a dibenzothiophene derivativeeach having the structure represented by the above General Formula (G1).Therefore, another embodiment of the present invention is a dibenzofuranderivative having a structure represented by Structural Formula (100)below.

Still another embodiment of the present invention is a dibenzothiophenederivative having a structure represented by Structural Formula (300)below.

The above heterocyclic compounds which are embodiments of the presentinvention represented by any of General Formulae (G1), (G2-1), (G3), and(G4) and Structural Formulae (100) and (300) can be preferably used as amaterial for a light-emitting element or an organic device such as anorganic transistor. Therefore, a light-emitting element including theabove heterocyclic compound is one embodiment of the present invention.

Another embodiment of the present invention is a light-emitting elementincluding a light-emitting layer that includes the above heterocycliccompound. The heterocyclic compound which is one embodiment of thepresent invention exhibits blue light emission and high emissionefficiency, and therefore can be preferably used as a material for alight-emitting layer of a light-emitting element.

Still another embodiment of the present invention is a light-emittingelement including a light-emitting layer that includes the aboveheterocyclic compound and a light-emitting substance. The heterocycliccompound which is one embodiment of the present invention has a wideenergy gap. Therefore, in a light-emitting element, such a heterocycliccompound achieves high emission efficiency, when used as a host materialin which the light-emitting substance of the light-emitting layer isdispersed. In particular, when the heterocyclic compound is used as ahost material for a blue light-emitting substance, a blue light-emittingelement having high emission efficiency can be provided.

Yet another embodiment of the present invention is a light-emittingelement having at least a light-emitting layer and a hole-transportlayer between a pair of electrodes, in which the hole-transport layerincludes the above heterocyclic compound. Since the heterocycliccompound of one embodiment of the present invention has a highhole-transport property, the heterocyclic compound can be preferablyused as a material for the hole-transport layer.

Since the light-emitting element of one embodiment of the presentinvention which is obtained as above can realize high emissionefficiency, a light-emitting device (such as an image display device)using this light-emitting element can realize low power consumption.Therefore, a light-emitting device using the above light-emittingelement is one embodiment of the present invention. In addition, anelectronic device and a lighting device using the light-emitting deviceare also embodiments of the present invention.

The light-emitting device in this specification cover an image displaydevice using a light-emitting element and also the following devices: amodule including a light-emitting element to which a connector such asan anisotropic conductive film, a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is added; a module in which the top of theTAB tape or the TCP is provided with a printed wiring board; a module inwhich an integrated circuit (IC) is directly mounted on a light-emittingelement by a chip on glass (COG) technique; and the like. Furthermore, alight-emitting device used in a lighting device and the like is alsoincluded.

One embodiment of the present invention can provide a novel heterocycliccompound that exhibits blue light emission. Further, the heterocycliccompound which is one embodiment of the present invention has highemission efficiency. Therefore, by using the heterocyclic compound ofone embodiment of the present invention for a light-emitting element,the light-emitting element can have high emission efficiency. Further,the use of the heterocyclic compound of one embodiment of the presentinvention enables the production of a light-emitting device, anelectronic device, and a lighting device in each of which powerconsumption is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B each illustrate a light-emitting element of oneembodiment of the present invention.

FIGS. 2A and 2B each illustrate a light-emitting element of oneembodiment of the present invention.

FIGS. 3A and 3B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 4A and 4B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 5A to 5D each illustrate an electronic device of one embodiment ofthe present invention.

FIG. 6 illustrates a lighting device according to one embodiment of oneembodiment of the present invention.

FIG. 7 illustrates a lighting device according to one embodiment of thepresent invention.

FIG. 8 illustrates a lighting device according to one embodiment of thepresent invention.

FIGS. 9A and 9B show ¹H NMR charts of4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 10A and 10B show an absorption spectrum and an emission spectrumof a toluene solution of4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 11A and 11B show an absorption spectrum and an emission spectrumof a thin film of 4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 12A and 12B show CV measurement results of4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 13A and 13B show ¹H NMR charts of4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene.

FIGS. 14A and 14B show an absorption spectrum and an emission spectrumof a toluene solution of4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene.

FIGS. 15A and 15B show an absorption spectrum and an emission spectrumof a thin film of 4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene.

FIGS. 16A and 16B show CV measurement results of4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene.

FIG. 17 shows luminance vs. current efficiency characteristics ofLight-emitting Element 1 and Reference Light-emitting Element 1.

FIG. 18 shows voltage vs. luminance characteristics of Light-emittingElement 1 and Reference Light-emitting Element 1.

FIG. 19 shows luminance vs. external quantum efficiency characteristicsof Light-emitting Element 1 and Reference Light-emitting Element 1.

FIG. 20 shows emission spectra of Light-emitting Element 1 and ReferenceLight-emitting Element 1.

FIG. 21 shows results of reliability tests of Light-emitting Element 1and Reference Light-emitting Element 1.

FIG. 22 shows luminance vs. current efficiency characteristics ofLight-emitting Element 2 and Reference Light-emitting Element 2.

FIG. 23 shows voltage vs. luminance characteristics of Light-emittingElement 2 and Reference Light-emitting Element 2.

FIG. 24 shows luminance vs. external quantum efficiency characteristicsof Light-emitting Element 2 and Reference Light-emitting Element 2.

FIG. 25 shows emission spectra of Light-emitting Element 2 and ReferenceLight-emitting Element 2.

FIG. 26 shows results of reliability tests of Light-emitting Element 2and Reference Light-emitting Element 2.

FIG. 27 shows luminance vs. current efficiency characteristics ofLight-emitting Element 3.

FIG. 28 shows voltage vs. luminance characteristics of Light-emittingElement 3.

FIG. 29 shows luminance vs. external quantum efficiency characteristicsof Light-emitting Element 3.

FIG. 30 shows an emission spectrum of Light-emitting Element 3.

FIG. 31 shows results of reliability tests of Light-emitting Element 3.

FIG. 32 shows luminance vs. current efficiency characteristics ofLight-emitting Element 4.

FIG. 33 shows voltage vs. luminance characteristics of Light-emittingElement 4.

FIG. 34 shows luminance vs. external quantum efficiency characteristicsof Light-emitting Element 4.

FIG. 35 shows an emission spectrum of Light-emitting Element 4.

FIG. 36 shows results of reliability tests of Light-emitting Element 4.

FIGS. 37A and 37B each illustrate a light-emitting element of Examples.

FIGS. 38A and 38B show ¹H NMR charts of4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran.

FIGS. 39A and 39B show an absorption spectrum and an emission spectrumof a toluene solution of4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran.

FIGS. 40A and 40B show an absorption spectrum and an emission spectrumof a thin film of4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran.

FIGS. 41A and 41B show CV measurement results of4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran.

FIG. 42 shows luminance vs. current efficiency characteristics ofLight-emitting Element 5.

FIG. 43 shows voltage vs. luminance characteristics of Light-emittingElement 5.

FIG. 44 shows luminance vs. external quantum efficiency characteristicsof Light-emitting Element 5.

FIG. 45 shows an emission spectrum of Light-emitting Element 5.

FIG. 46 shows results of reliability tests of Light-emitting Element 5.

FIGS. 47A and 47B show ¹H NMR charts of2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 48A and 48B show an absorption spectrum and an emission spectrumof a toluene solution of2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 49A and 49B show an absorption spectrum and an emission spectrumof a thin film of 2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIGS. 50A and 50B show CV measurement results of2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran.

FIG. 51 shows luminance vs. current efficiency characteristics ofLight-emitting Element 6.

FIG. 52 shows voltage vs. luminance characteristics of Light-emittingElement 6.

FIG. 53 shows luminance vs. external quantum efficiency characteristicsof Light-emitting Element 6.

FIG. 54 shows an emission spectrum of Light-emitting Element 6.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings. Note that the invention is notlimited to the description below, and it will be easily understood bythose skilled in the art that various changes and modifications can bemade without departing from the spirit and scope of the invention.Therefore, the invention should not be construed as being limited to thedescription in the following embodiments.

Embodiment 1

In Embodiment 1, a heterocyclic compound of one embodiment of thepresent invention is described.

One embodiment of the present invention is the heterocyclic compoundrepresented by General Formula (G1).

In General Formula (G1), Ar¹ and Ar² independently represent asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, R¹to R¹¹ independently represent any of hydrogen, an alkyl group having 1to 4 carbon atoms, and a substituted or unsubstituted aryl group having6 to 13 carbon atoms, and A represents a substituent represented byGeneral Formula (S1) or (S2). In General Formulae (S1) and (S2), Xrepresents oxygen or sulfur, and R¹² to R¹⁸ independently represent anyof hydrogen, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.Note that the carbon atoms in an aryl group in this specification meancarbon atoms which form a ring of the main skeleton, not includingcarbon atoms in a substituent bonded to the main skeleton.

Another embodiment of the present invention is a heterocyclic compound(G2-1) in which, in General Formula (G1), A is the substituentrepresented by General Formula (S1).

Because of its easy synthesis, preferred is a heterocyclic compound (G3)of one embodiment of the present invention in which, in General Formula(G2-1), R⁸ to R¹¹ are each substituted with hydrogen.

Because of its easy synthesis, further preferred is a heterocycliccompound (G4) of one embodiment of the present invention in which, inGeneral Formula (G2-1), R¹ to R¹⁸ are each substituted with hydrogen.

Still another embodiment of the present invention is a heterocycliccompound (G2-2) in which, in General Formula (G1), A is the substituentrepresented by General Formula (S2).

As specific structures of Ar¹ and Ar² in General Formula (G1), there aresubstituents represented by Structural Formulae (1-1) to (1-16).

As specific structures of R¹ to R¹¹ in General Formula (G1), there aresubstituents represented by Structural Formulae (2-1) to (2-9) inaddition to Structural Formulae (1-1) to (1-16) above.

As specific structures of R¹² to R¹⁸ in General Formulae (S1) and (S2),there are substituents represented by the above-described StructuralFormulae (1-1) to (1-16) and (2-1) to (2-9).

Specific examples of the heterocyclic compound represented by GeneralFormula (G1) include, but not limited to, dibenzofuran derivativesrepresented by Structural Formulae (100) to (203) and dibenzothiophenederivatives represented by Structural Formulae (300) to (400).

A variety of reactions can be applied to a synthesis method of theheterocyclic compound which is one embodiment of the present invention.For example, synthesis reactions described below enable the synthesis ofthe heterocyclic compound of one embodiment of the present inventionrepresented by General Formula (G1). Synthesis Method 1 is a method ofsynthesizing the heterocyclic compound (G2-1) of one embodiment of thepresent invention in which, in General Formula (G1), A is thesubstituent represented by General Formula (S1). Further, SynthesisMethod 2 is a method of synthesizing the heterocyclic compound (G2-2) ofone embodiment of the present invention in which, in General Formula(G1), A is the substituent represented by General Formula (S2). Notethat the synthesis methods of the heterocyclic compound which is oneembodiment of the present invention are not limited to the synthesismethods below.

<Synthesis Method 1 of Heterocyclic Compound Represented by GeneralFormula (G1)>

First, Synthesis Scheme (A-1) will be shown.

The heterocyclic compound (G2-1) of one embodiment of the presentinvention can be synthesized according to Synthesis Scheme (A-1).Specifically, a halogen compound of an anthracene derivative (CompoundA) is coupled with an organoboron compound of a dibenzofuran derivativeor a dibenzothiophene derivative (Compound B1) according to aSuzuki-Miyaura reaction using a palladium catalyst, whereby theheterocyclic compound (Compound G2-1) described in this embodiment canbe provided.

In Synthesis Scheme (A-1), X represents oxygen or sulfur. In SynthesisScheme (A-1), D represents a halogen. As the halogen, iodine or bromineis preferable.

In Synthesis Scheme (A-1), R¹⁰¹ and R¹⁰² independently representhydrogen or an alkyl group having 1 to 6 carbon atoms, may be the sameor different from each other, and may be combined with each other toform a ring.

Examples of the palladium catalyst that can be used in Synthesis Scheme(A-1) include palladium(II) acetate,tetrakis(triphenylphosphine)palladium(0), and the like. Examples of aligand of the palladium catalyst which can be used in Synthesis Scheme(A-1) include tri(ortho-tolyl)phosphine, triphenylphosphine,tricyclohexylphosphine, and the like.

Examples of a base that can be used in Synthesis Scheme (A-1) include anorganic base such as sodium tert-butoxide, an inorganic base such aspotassium carbonate, and the like.

Examples of solvents that can be used in Synthesis Scheme (A-1) includea mixed solvent of toluene and water; a mixed solvent of toluene,alcohol such as ethanol, and water; a mixed solvent of xylene and water;a mixed solvent of xylene, alcohol such as ethanol, and water; a mixedsolvent of benzene and water; a mixed solvent of benzene, alcohol suchas ethanol, and water; a mixed solvent of an ether such as1,2-dimethoxyethane and water; and the like. Use of a mixed solvent oftoluene and water or a mixed solvent of toluene, ethanol, and water ismore preferable.

Thus, the heterocyclic compound of this embodiment can be synthesized.

<Synthesis Method 2 of Heterocyclic Compound Represented by GeneralFormula (G1)>

First, Synthesis Scheme (B-1) will be shown.

The heterocyclic compound (G2-2) of one embodiment of the presentinvention can be synthesized according to Synthesis Scheme (B-1).Specifically, a halogen compound of an anthracene derivative (CompoundA) is coupled with an organoboron compound of a dibenzofuran derivativeor a dibenzothiophene derivative (Compound B2) according to aSuzuki-Miyaura reaction using a palladium catalyst, whereby theheterocyclic compound (Compound G2-2) described in this embodiment canbe provided.

In Synthesis Scheme (B-1), X represents oxygen or sulfur. In SynthesisScheme (B-1), D represents a halogen. As the halogen, iodine or bromineis preferable.

In Synthesis Scheme (B-1), R¹⁰¹ and R¹⁰² independently representhydrogen or an alkyl group having 1 to 6 carbon atoms, may be the sameor different from each other, and may be combined with each other toform a ring.

Examples of the palladium catalyst that can be used in Synthesis Scheme(B-1) include palladium(II) acetate,tetrakis(triphenylphosphine)palladium(0), and the like. Examples of aligand of the palladium catalyst which can be used in Synthesis Scheme(B-1) include tri(ortho-tolyl)phosphine, triphenylphosphine,tricyclohexylphosphine, and the like.

Examples of a base that can be used in Synthesis Scheme (B-1) include anorganic base such as sodium tert-butoxide, an inorganic base such aspotassium carbonate, and the like.

Examples of solvents that can be used in Synthesis Scheme (B-1) includea mixed solvent of toluene and water; a mixed solvent of toluene,alcohol such as ethanol, and water; a mixed solvent of xylene and water;a mixed solvent of xylene, alcohol such as ethanol, and water; a mixedsolvent of benzene and water; a mixed solvent of benzene, alcohol suchas ethanol, and water; a mixed solvent having an ether such as1,2-dimethoxyethane and water; and the like. Use of a mixed solvent oftoluene and water or a mixed solvent of toluene, ethanol, and water ismore preferable.

Thus, the heterocyclic compound of this embodiment can be synthesized.

The heterocyclic compound of this embodiment emits blue light and has ahole-transport property. Also, the heterocyclic compound of thisembodiment exhibits high emission efficiency. Accordingly, with the useof the heterocyclic compound of this embodiment for a light-emittingelement, the light-emitting element can exhibit high emissionefficiency. Further, the use of the heterocyclic compound of thisembodiment can provide a light-emitting device, an electronic device,and a lighting device each having reduced power consumption.

Embodiment 2

In Embodiment 2, a light-emitting element in which the heterocycliccompound of one embodiment of the present invention is used for an ELlayer will be described with reference to FIGS. 1A and 1B.

In the light-emitting element of this embodiment, the EL layer having atleast a light-emitting layer is interposed between a pair of electrodes.The EL layer may also have a plurality of layers in addition to thelight-emitting layer. The plurality of layers is a combination of layersthat include a substance having a high carrier-injection property and asubstance having a high carrier-transport property. Those layers arestacked so that a light-emitting region is formed in a region away fromthe electrodes, that is, carriers recombine in a region away from theelectrodes. In this specification, the layer that includes a substancehaving a high carrier-injection property or a substance having a highcarrier-transport property is also called a functional layer whichfunctions, for instance, to inject or transport carriers. As thefunctional layer, a hole-injection layer, a hole-transport layer, anelectron-injection layer, an electron-transport layer, or the like canbe used.

In the light-emitting element of this embodiment illustrated in FIG. 1A,an EL layer 102 having a light-emitting layer 113 is provided between apair of electrodes, a first electrode 101 and a second electrode 103.The EL layer 102 includes a hole-injection layer 111, a hole-transportlayer 112, the light-emitting layer 113, an electron-transport layer114, and an electron-injection layer 115. The light-emitting element inFIG. 1A includes: the first electrode 101 formed over a substrate 100;the hole-injection layer 111, the hole-transport layer 112, thelight-emitting layer 113, the electron-transport layer 114, and theelectron-injection layer 115 which are stacked over the first electrode101 in this order; and the second electrode 103 provided over theelectron-injection layer 115. Note that, in the light-emitting elementdescribed in this embodiment, the first electrode 101 functions as ananode and the second electrode 103 functions as a cathode.

The substrate 100 is used as a support of the light-emitting element.For example, glass, quartz, plastic, or the like can be used for thesubstrate 100. Alternatively, a flexible substrate may be used. Theflexible substrate is a substrate that can be bent, such as a plasticsubstrate made of polycarbonate, polyarylate, or polyether sulfone, forexample. Alternatively, a film (made of polypropylene, polyester, vinyl,polyvinyl fluoride, vinyl chloride, or the like), an inorganic filmformed by evaporation, or the like can be used. Note that materialsother than glass and plastic can be used as long as they can function asa support of the light-emitting element.

For the first electrode 101, a metal, an alloy, an electricallyconductive compound, a mixture thereof, or the like which has a highwork function (specifically, a work function of 4.0 eV or more) ispreferably used. Specific examples include indium tin oxide (ITO),indium tin oxide containing silicon or silicon oxide, indium zinc oxide(IZO), indium oxide containing tungsten oxide and zinc oxide (IWZO), andthe like. Films of these conductive metal oxides are usually formed bysputtering; however, a sol-gel method or the like may also be used. Forexample, indium oxide-zinc oxide (IZO) can be formed by a sputteringmethod using a target in which 1 wt % to 20 wt % of zinc oxide is addedto indium oxide. IWZO can be formed by a sputtering method using atarget in which 0.5 wt % to 5 wt % of tungsten oxide and 0.1 wt % to 1wt % of zinc oxide are added to indium oxide. Further, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitrides ofmetal materials (e.g., titanium nitride), and the like can be given.

Note that, in the EL layer 102, when a layer in contact with the firstelectrode 101 is formed using a composite material in which an organiccompound and an electron acceptor (acceptor) described later are mixed,the first electrode 101 can be formed using any of a variety of metals,alloys, and electrically conductive compounds, a mixture thereof, andthe like regardless of the work function. For example, aluminum (Al),silver (Ag), an alloy containing aluminum (e.g., Al—Si), or the like canbe used.

The EL layer 102 formed over the first electrode 101 includes at leastthe light-emitting layer 113, and part of the EL layer 102 is formedusing the heterocyclic compound which is one embodiment of the presentinvention. For the part of the EL layer 102, a known substance can beused, and either a low molecular compound or a high molecular compoundcan be used. Note that the substance used for forming the EL layer 102may have not only a structure formed of only an organic compound butalso a structure in which an inorganic compound is partially contained.

As illustrated in FIGS. 1A and 1B, the EL layer 102 is formed bystacking as appropriate the hole-injection layer 111, the hole-transportlayer 112, the electron-transport layer 114, the electron-injectionlayer 115, and the like in combination as well as the light-emittinglayer 113.

The hole-injection layer 111 includes a substance having a highhole-injection property. As the substance having a high hole-injectionproperty, for example, metal oxides such as molybdenum oxide, titaniumoxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide,zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungstenoxide, and manganese oxide can be used. Alternatively, aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc), copper(II) phthalocyanine (abbreviation: CuPc), or vanadylphthalocyanine (abbreviation: VOPc) can be used.

Further, as examples of low molecular organic compounds, there arearomatic amine compounds such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Further alternatively, any of high molecular compounds (e.g., oligomers,dendrimers, or polymers) can be used. Examples of the high molecularcompounds include poly(N-vinylcarbazole) (abbreviation: PVK),poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA),poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD), and the like. Alternatively, a high molecular compound towhich acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or polyaniline/poly(styrenesulfonic acid) (PAni/PSS), can be used.

For the hole-injection layer 111, a composite material in which anorganic compound and an electron acceptor (acceptor) are mixed may beused. Such a composite material is excellent in a hole-injectionproperty and a hole-transport property because holes are generated inthe organic compound by the electron acceptor. In this case, the organiccompound is preferably a material excellent in transporting thegenerated holes (a substance having a high hole-transport property).

As the organic compound for the composite material, a variety ofcompounds such as an aromatic amine compound, a carbazole derivative,aromatic hydrocarbon, and a high molecular compound (such as oligomer,dendrimer, or polymer) can be used. The organic compound used for thecomposite material is preferably an organic compound having a highhole-transport property. Specifically, a substance having a holemobility of 10⁻⁶ cm²/Vs or more is preferably used. Note that asubstance other than the above may be used as long as it has ahole-transport property higher than its electron-transport property. Theorganic compounds which can be used for the composite material arespecifically shown below.

For example, as the organic compounds that can be used for the compositematerial, there are aromatic amine compounds such as TDATA, MTDATA,DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), andN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); and carbazole derivatives such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl-2,3,5,6-tetraphenylbenzene.

Alternatively, any of the following aromatic hydrocarbon compounds canbe used: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, and the like.

Still alternatively, any of the following aromatic hydrocarbon compoundscan be used: 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene,9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

As electron acceptors, organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, and a transition metal oxide can be given. Inaddition, oxides of metals belonging to Groups 4 to 8 in the periodictable can be also given. Specifically, vanadium oxide, niobium oxide,tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide are preferable since theirelectron-accepting property is high. Among these, molybdenum oxide isespecially preferable since it is stable in the air and its hygroscopicproperty is low and is easily treated.

Note that the hole-injection layer 111 may be formed using a compositematerial of the above-described high molecular compound, such as PVK,PVTPA, PTPDMA, or Poly-TPD, and the above-described electron acceptor.

The hole-transport layer 112 includes a substance having a highhole-transport property. The heterocyclic compound of one embodiment ofthe present invention described in Embodiment 1 has an excellenthole-transport property and therefore can be preferably used for thehole-transport layer 112.

As the substance having a high hole-transport property, it is possibleto use an aromatic amine compound such as NPB, TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), for example. The materials described here aremainly materials having a hole mobility of 10⁻⁶ cm²/Vs or more. Notethat a substance other than the above may be used as long as it has ahole-transport property higher than its electron-transport property. Thelayer containing a substance having a high hole-transport property isnot limited to a single layer, and a stacked layer in which two or morelayers containing the above-described substance are stacked may be used.

Alternatively, for the hole-transport layer 112, a high molecularcompound such as PVK, PVTPA, PTPDMA, or Poly-TPD can be used.

The light-emitting layer 113 includes a light-emitting substance. As thelight-emitting substance, for example, a fluorescent compound whichexhibits fluorescence or a phosphorescent compound which exhibitsphosphorescence can be used, other than the heterocyclic compound of oneembodiment of the present invention described in Embodiment 1.

The phosphorescent compounds that can be used for the light-emittinglayer 113 will be given. Examples of the materials for blue lightemission includeN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA), and the like. In addition, examples of thematerials for green light emission includeN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), and the like. Further, examples of thematerials for yellow light emission include rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),and the like. Furthermore, examples of the materials for red lightemission includeN,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like.

In addition, the phosphorescent compounds that can be used for thelight-emitting layer 113 will be given. Examples of the materials forblue light emission includebis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(II)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate(abbreviation: Fir(acac)), and the like. Examples of the materials forgreen light emission includetris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(ppy)₂(acac)),bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate(abbreviation: Ir(pbi)₂(acac)),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)), and the like. Examples of the materials for yellowlight emission includebis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-(perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)), and the like. Examples of the materialsfor orange light emission includetris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),bis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(pq)₂(acac)), and the like. Examples of the materialsfor red light emission include organometallic complexes such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′))iridium(III)acetylacetonate(abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)), and2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II)(abbreviation: PtOEP). Furthermore, since light emission from a rareearth metal ion (electron transition between different multiplicities)can be obtained by rare earth metal complexes such astris(acetylacetonatoXmonophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)), such rare earth metal complexes can beused as a phosphorescent compound.

Note that the light-emitting layer 113 may have a structure in which anyof the above light-emitting substances (a guest material) is dispersedin another substance (a host material). By using the heterocycliccompound of one embodiment of the present invention for thelight-emitting layer 113, the light-emitting layer 113 can be alight-emitting layer having a high hole-transport property. In thelight-emitting layer 113, the heterocyclic compound of one embodiment ofthe present invention described in Embodiment 1 can be used as a hostmaterial, and a guest material which is a light-emitting substance isdispersed in the heterocyclic compound of Embodiment 1; in this manner,it is possible to obtain light emission from the guest material.

When the heterocyclic compound of one embodiment of the presentinvention is used as a host material (a substance in which alight-emitting substance different from the host material is dispersed),the emission color of the light-emitting substance can be obtained. Itis also possible to obtain the mixed color of the emission color of theheterocyclic compound of one embodiment of the present invention and theemission color of the light-emitting substance dispersed in thisheterocyclic compound.

As the substance in which a light-emitting substance is dispersed, avariety of substances can be used other than the heterocyclic compoundof one embodiment of the present invention described in Embodiment 1. Itis preferable to use a substance whose lowest unoccupied molecularorbital (LUMO) level is higher than that of the light-emitting substanceand whose highest occupied molecular orbital (HOMO) level is lower thanthat of the light-emitting substance.

As the substance in which the light-emitting substance is dispersed,specifically, any of the following materials can be used as analternative to the heterocyclic compound of one embodiment of thepresent invention: metal complexes such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), andbathocuproine (BCP); condensed aromatic compounds such as9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3),9,10-diphenylanthracene (abbreviation: DPAnth), and6,12-dimethoxy-5,11-diphenylchrysene; aromatic amine compounds such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, and BSPB; or thelike.

As the substance (host material) in which the light-emitting substance(guest material) is dispersed, plural kinds of substances can be used.

As the light-emitting substance, a high molecular compound can also beused. Specifically, examples of the materials for blue light emissioninclude poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: PFO),poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation: PF-DMOP),poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH), and the like. Further, examples of thematerials for green light emission include poly(p-phenylenevinylene)(abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazole-4,7-diyl)](abbreviation:PFBT),poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)],and the like. Furthermore, examples of the materials for orange to redlight emission includepoly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene](abbreviation:MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation: R4-PAT),poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]},poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD), and the like.

The electron-transport layer 114 includes a substance having a highelectron-transport property. The electron-transport layer 114 is formedusing, for example, a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq). Alternatively, a metal complex or the like including anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂) canbe used. Other than the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances described here are mainly materials having an electronmobility of 10⁻⁶ cm²/Vs or more. Further, the electron-transport layeris not limited to a single layer, and a stacked layer in which two ormore layers containing any of the above-described substances are stackedmay be used.

The electron-injection layer 115 includes a substance having a highelectron-injection property. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium (Li), cesium (Cs), calcium (Ca), lithium fluoride (LiF), cesiumfluoride (CsF), calcium fluoride (CaF₂), or lithium oxide (LiO_(x)), canbe used. Alternatively, a rare earth metal compound like erbium fluoride(ErF₃) can be used. Alternatively, the above-mentioned substances forforming the electron-transport layer 114 can also be used.

Alternatively, a composite material in which an organic compound and anelectron donor (donor) are mixed may be used for the electron-injectionlayer 115. Such a composite material is excellent in anelectron-injection property and an electron-transport property becauseelectrons are generated in the organic compound by the electron donor.In this case, the organic compound is preferably a material excellent intransporting the generated electrons. Specifically, for example, thesubstances for forming the electron-transport layer 114 (e.g., a metalcomplex and a heteroaromatic compound), which are described above, canbe used. As the electron donor, a substance showing an electron-donatingproperty with respect to the organic compound may be used. Specifically,an alkali metal, an alkaline earth metal, and a rare earth metal arepreferable, and Li, Cs, magnesium (Mg), Ca, erbium (Er), ytterbium (Yb),and the like are given. In addition, alkali metal oxide or alkalineearth metal oxide such as lithium oxide, calcium oxide, barium oxide,and the like can be given. Lewis base such as magnesium oxide canalternatively be used. An organic compound such as tetrathiafulvalene(abbreviation: TTF) can alternatively be used.

Note that each of the above-described hole-injection layer 111,hole-transport layer 112, light-emitting layer 113, electron-transportlayer 114, and electron-injection layer 115 can be formed by a methodsuch as an evaporation method (which includes a vacuum evaporationmethod), an inkjet method, or a coating method.

When the second electrode 103 functions as a cathode, it can be formedusing a metal, an alloy, an electrically-conductive compound, a mixturethereof, or the like having a low work function (preferably, a workfunction of 3.8 eV or less). Specifically, Al, Ag, or the like can beused besides an element belonging to Group 1 or Group 2 of the periodictable, that is, an alkali metal such Li or Cs and an alkaline earthmetal such as magnesium (Mg), calcium (Ca), or strontium (Sr); an alloyof the above metals (e.g., Mg—Ag or Al—Li); a rare earth metal such aseuropium (Eu) or ytterbium (Yb); an alloy of the above metals, or thelike.

Note that, in the case where in the EL layer 102, a layer formed incontact with the second electrode 103 is formed using a compositematerial in which the organic compound and the electron donor (donor),which are described above, are mixed, a variety of conductive materialssuch as Al, Ag, ITO, and indium tin oxide containing silicon or siliconoxide can be used regardless of the work function.

Note that the second electrode 103 can be formed by a vacuum evaporationmethod or a sputtering method. Alternatively, in the case of using asilver paste or the like, a coating method, an inkjet method, or thelike can be used.

In the above-described light-emitting element of this embodiment, acurrent flows due to a potential difference generated between the firstelectrode 101 and the second electrode 103, and holes and electronsrecombine in the EL layer 102, so that light is emitted. Then, thisemitted light is extracted out through one or both of the firstelectrode 101 and the second electrode 103. Therefore, one of or boththe first electrode 101 and the second electrode 103 is/are an electrodehaving the property of transmitting visible light.

Further, a structure of a layer provided between the first electrode 101and the second electrode 103 is not limited to the above describedstructure. A structure other than the above may alternatively beemployed as long as a light-emitting region in which holes and electronsrecombine is provided in a portion away from the first electrode 101 andthe second electrode 103 in order to prevent quenching due to proximityof the light-emitting region to a metal.

In other words, a layered structure of the layer is not particularlylimited, and a layer formed using a substance having a highelectron-transport property, a substance having a high hole-transportproperty, a substance having a high electron-injection property, asubstance having a high hole-injection property, a bipolar substance (asubstance having a high electron-transport property and a highhole-transport property), a hole blocking material, or the like mayfreely be combined with a light-emitting layer including theheterocyclic compound described in Embodiment 1.

In a light-emitting element illustrated in FIG. 1B, the EL layer 102 isprovided between the first electrode 101 and the second electrode 103over the substrate 100. The EL layer 102 includes the hole-injectionlayer 111, the hole-transport layer 112, the light-emitting layer 113,the electron-transport layer 114, and the electron-injection layer 115.The light-emitting element in FIG. 1B includes: the second electrode 103serving as a cathode over the substrate 100; the electron-injectionlayer 115, the electron-transport layer 114, the light-emitting layer113, the hole-transport layer 112, and the hole-injection layer 111which are stacked over the second electrode 103 in this order; and thefirst electrode 101 serving as an anode over the hole-injection layer111.

A method of forming a light-emitting element will now be specificallydescribed.

The light-emitting element of this embodiment has a structure in whichan EL layer is interposed between a pair of electrodes. The EL layer atleast has a light-emitting layer and is formed using the heterocycliccompound described in Embodiment 1. Further, the EL layer may include afunctional layer (e.g., a hole-injection layer, a hole-transport layer,an electron-transport layer, or an electron-injection layer) in additionto the light-emitting layer. Each electrode (the first electrode or thesecond electrode), the light-emitting layer, and each functional layermay be formed by any of the wet processes such as a droplet dischargingmethod (an inkjet method), a spin coating method, or a printing method,or by a dry process such as a vacuum evaporation method, a CVD method,or a sputtering method. A wet process allows formation at atmosphericpressure with a simple device and process, thereby having the effects ofsimplifying the process and improving the productivity. In contrast, adry process does not need dissolution of a material to enable use of amaterial that has low solubility in a solution, thereby expanding therange of material choices.

All the thin films included in the light-emitting element may be formedby a wet method. In this case, the light-emitting element can bemanufactured with only facilities needed for a wet process.Alternatively, formation of the stacked layers up to formation of thelight-emitting layer may be performed by a wet process whereas thefunctional layer, the first electrode, and the like which are stackedover the light-emitting layer may be formed by a dry process. Furtheralternatively, the second electrode and the functional layer may beformed by a dry process before the formation of the light-emitting layerwhereas the light-emitting layer, the functional layer stackedthereover, and the first electrode may be formed by a wet process.Needless to say, this embodiment is not limited to this, and thelight-emitting element can be formed by appropriate selection from a wetmethod and a dry method depending on a material to be used, necessaryfilm thickness, and the interface state.

In this embodiment, the light-emitting element is fabricated over asubstrate made of glass, plastic or the like. By forming a plurality ofsuch light-emitting elements over one substrate, a passive matrixlight-emitting device can be manufactured. Alternatively, a thin filmtransistor (TFT), for instance, may be formed over a substrate formed ofglass, plastic, or the like, and a light-emitting element may befabricated over an electrode electrically connected to the TFT; thus, anactive matrix light-emitting device in which the TFT controls the driveof the light-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT. Either a staggeredTFT or an inverted staggered TFT may be employed. In addition,crystallinity of a semiconductor used for the TFT is not particularlylimited either; an amorphous semiconductor or a crystallinesemiconductor may be used. In addition, a driver circuit formed over aTFT substrate may be constructed from both n-channel and p-channel TFTsor from one of n-channel and p-channel TFTs.

The heterocyclic compound of one embodiment of the present inventiondescribed in Embodiment 1 has a high hole-transport property and highemission efficiency. Accordingly, by using the heterocyclic compounddescribed in Embodiment 1 for a light-emitting element, thelight-emitting element can exhibit high emission efficiency.

Since the light-emitting element of an embodiment of the presentinvention thus obtained has high emission efficiency, a light-emittingdevice (such as an image display device) that uses this light-emittingelement can realize low power consumption.

Note that by use of the light-emitting element described in thisembodiment, a passive matrix light-emitting device or an active matrixlight-emitting device in which the driving of the light-emitting elementis controlled by a TFT can be manufactured.

In this embodiment, the structures can be combined with those of theother embodiments, as appropriate.

Embodiment 3

In Embodiment 3, a mode of a light-emitting element having a structurein which a plurality of light-emitting units is stacked (hereinafter,referred to as a stacked-type element) will be described with referenceto FIGS. 2A and 2B. This light-emitting element is a light-emittingelement including a plurality of light-emitting units between a firstelectrode and a second electrode.

In FIG. 2A, a first light-emitting unit 311 and a second light-emittingunit 312 are stacked between a first electrode 301 and a secondelectrode 303. In this embodiment, the first electrode 301 functions asan anode and the second electrode 303 functions as a cathode. The firstelectrode 301 and the second electrode 303 can be the same as those inEmbodiment 2. The structures of the first light-emitting unit 311 andthe second light-emitting unit 312 may be the same or different fromeach other, and can be the same as those described in Embodiment 2.

Further, a charge generating layer 313 is provided between the firstlight-emitting unit 311 and the second light-emitting unit 312. Thecharge generation layer 313 functions so that electrons are injectedinto one light-emitting unit and holes are injected into the otherlight-emitting unit by application of a voltage between the firstelectrode 301 and the second electrode 303. In this embodiment, when avoltage is applied to the first electrode 301 so that the potentialthereof is higher than that of the second electrode 303, the chargegeneration layer 313 injects electrons into the first light-emittingunit 311 and injects holes into the second light-emitting unit 312.

Note that the charge generation layer 313 preferably has the property oftransmitting visible light in terms of light extraction efficiency.Further, the charge generation layer 313 functions even when it haslower conductivity than the first electrode 301 or the second electrode303.

The charge generation layer 313 may have either a structure including anorganic compound having a high hole-transport property and an electronacceptor or a structure including an organic compound having a highelectron-transport property and an electron donor. Alternatively, bothof these structures may be stacked.

In the case where the charge generation layer 313 contains an electronacceptor and an organic compound having a high hole-transport property,as the organic compound having a high hole-transport property, forexample, the heterocyclic compound of one embodiment of the presentinvention, an aromatic amine compound such as NPB, TPD, TDATA, MTDATA,or 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), or the like can be used. The materials describedhere are mainly materials having a hole mobility of 10⁻⁶ cm²/Vs or more.Note that a substance other than the above may be used as long as it isan organic compound having a hole-transport property higher than itselectron-transport property.

Further, as the electron acceptor,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. Moreover, oxides of metals belonging toGroups 4 to 8 of the periodic table can be used. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferablebecause they have a high electron-accepting property. Among these metaloxides, molybdenum oxide, which is easy to handle, is preferred becauseof its stability in air and low hygroscopic property.

On the other hand, in the case where the charge generation layer 313includes an electron donor and an organic compound having a highhole-transport property, as the organic compound having a highelectron-transport property, for example, a metal complex having aquinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq₃,BeBq₂, or BAlq, or the like can be used. Alternatively, a metal complexhaving an oxazole-based ligand or a thiazole-based ligand, such asZn(BOX)₂ or Zn(BTZ)₂ can be used. Alternatively, in addition to such ametal complex, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. Thematerials described here are mainly materials having an electronmobility of 10⁻⁶ cm²/Vs or more. Note that a substance other than theabove may be used as long as it is an organic compound having anelectron-transport property higher than its hole-transport property.

Further, as the electron donor, an alkali metal, an alkaline earthmetal, a rare earth metal, a metal belonging to Group 13 of the periodictable, or an oxide or carbonate thereof can be used. Specifically, Li,Cs, Mg, Ca, Yb, indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used as the electron donor.

Note that by formation of the charge generation layer 313 using any ofthe above materials, it is possible to suppress an increase in drivevoltage caused by stacking the EL layers.

In this embodiment, the light-emitting element having two light-emittingunits is described; however, one embodiment of the present invention canbe similarly applied to a light-emitting element in which three or morelight-emitting units are stacked as illustrated in FIG. 2B. Byarrangement of a plurality of light-emitting units, which arepartitioned by the charge-generation layer between a pair of electrodes,as in the light-emitting element of this embodiment, light emission in ahigh luminance region can be achieved with current density kept low.Thus, its current density can be kept low, so that a light-emittingelement having a long lifetime can be realized.

With light-emitting units having emission colors different from eachother, the light-emitting element can be made to exhibit light emissionof a desired color as a whole. For example, in a light-emitting elementhaving two light-emitting units, the emission colors of the firstlight-emitting unit and the second light-emitting unit are madecomplementary; thus, the light-emitting element which emits white lightas a whole can be obtained. Note that the word “complementary” meanscolor relationship in which an achromatic color is obtained when colorsare mixed. That is, white light emission can be obtained by mixture oflight obtained from substances emitting the lights of complementarycolors. The same can be applied to a light-emitting element which hasthree light-emitting units. For example, the light-emitting element as awhole can provide white light emission when the emission color of thefirst light-emitting unit is red, the emission color of the secondlight-emitting unit is green, and the emission color of the thirdlight-emitting unit is blue.

Note that this embodiment can be combined with any other embodiment asappropriate.

Embodiment 4

In Embodiment 4, a light-emitting device having a light-emitting elementof one embodiment of the present invention will be described withreference to FIGS. 3A and 3B. FIG. 3A is a top view illustrating alight-emitting device while FIG. 3B is a cross-sectional view takenalong lines A-B and C-D of FIG. 3A.

In FIG. 3A, reference numeral 401 denotes a driver circuit portion (asource side driver circuit), reference numeral 402 denotes a pixelportion, and reference numeral 403 denotes a driver circuit portion (agate side driver circuit), which are shown by a dotted line. Referencenumeral 404 denotes a sealing substrate, reference numeral 405 denotes asealant, and a portion enclosed by the sealant 405 is a space 407.

Note that a lead wiring 408 is a wiring for transmitting signals thatare to be inputted to the source side driver circuit 401 and the gateside driver circuit 403, and receives a video signal, a clock signal, astart signal, a reset signal, and the like from a flexible printedcircuit (FPC) 409 which serves as an external input terminal. Althoughonly the FPC is illustrated here, a printed wiring board (PWB) may beattached to the FPC. The light-emitting device in this specificationincludes not only a light-emitting device itself but also alight-emitting device to which an FPC or a PWB is attached.

Next, a cross-sectional structure will be described with reference toFIG. 3B. The driver circuit portion and the pixel portion are formedover an element substrate 410. In this case, one pixel in the pixelportion 402 and the source side driver circuit 401 which is the drivercircuit portion are illustrated.

A CMOS circuit, which is a combination of an n-channel TFT 423 with ap-channel TFT 424, is formed as the source side driver circuit 401. Sucha driver circuit may be any of a variety of circuits formed using TFTs,such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although adriver-integrated type in which a driver circuit is formed over thesubstrate is described in this embodiment, the present invention is notlimited to this type, and the driver circuit can be formed outside thesubstrate.

The pixel portion 402 includes a plurality of pixels having a switchingTFT 411, a current control TFT 412, and a first electrode 413electrically connected to a drain of the current control TFT 412. Notethat an insulator 414 is formed to cover an end portion of the firstelectrode 413. Here, the insulator 414 is formed using a positive typephotosensitive acrylic resin film.

In order to improve the coverage, the insulator 414 is provided suchthat either an upper end portion or a lower end portion of the insulator414 has a curved surface with a curvature. For example, when positivephotosensitive acrylic is used as a material for the insulator 414, onlyan upper end portion of the insulator 414 can have a curved surface witha radius of curvature (0.2 μm to 3 μm). Alternatively, the insulator 414can be formed using either a negative type that becomes insoluble in anetchant by light irradiation or a positive type that becomes soluble inan etchant by light irradiation.

A light-emitting layer 416 and a second electrode 417 are formed overthe first electrode 413. Here, as a material for forming the firstelectrode 413 functioning as the anode, it is preferable to use amaterial having a high work function. For example, a single layer of anITO film, an indium tin oxide film that includes silicon, an indiumoxide film that includes 2 wt % to 20 wt % of zinc oxide (ZnO), atitanium nitride film, a Cr film, a W film, a (zinc) Zn film, a Pt film,or the like, a stacked layer of a titanium nitride film and a film thatmainly includes aluminum, a three-layer structure of a titanium nitridefilm, a film that mainly includes aluminum and a titanium nitride film,or the like can be used. Note that, when a stacked structure isemployed, resistance of a wiring is low and a favorable ohmic contact isobtained.

In addition, the light-emitting layer 416 is formed by any of variousmethods such as an evaporation method using an evaporation mask, adroplet discharging method like an inkjet method, a printing method, anda spin coating method. The light-emitting layer 416 includes theheterocyclic compound described in Embodiment 1. Further, thelight-emitting layer 416 may include another material such as a lowmolecular material, an oligomer, a dendrimer, or a high molecularmaterial.

As a material used for the second electrode 417 which is formed over thelight-emitting layer 416 and serves as a cathode, it is preferable touse a material having a low work function (e.g., Al, Mg, Li, Ca, or analloy or compound thereof such as MgAg, Mg—In, Al—Li, LiF, or CaF₂). Inthe case where light generated in the light-emitting layer 416 istransmitted through the second electrode 417, the second electrode 417may be formed of a stack of a metal thin film having a reduced thicknessand a transparent conductive film (e.g., ITO, indium oxide containing 2wt % to 20 wt % of zinc oxide, indium tin oxide containing silicon orsilicon oxide, or zinc oxide (ZnO)).

The sealing substrate 404 is attached to the element substrate 410 withthe sealant 405; thus, a light-emitting element 418 is provided in thespace 407 enclosed by the element substrate 410, the sealing substrate404, and the sealant 405. Note that the space 407 is filled with afiller such as an inert gas (e.g., nitrogen or argon) or the sealant405.

Note that as the sealant 405, an epoxy-based resin is preferably used. Amaterial used for these is desirably a material which does not transmitmoisture or oxygen as possible. As a material for the sealing substrate404, a glass substrate, a quartz substrate, or a plastic substrateincluding fiberglass-reinforced plastics (FRP), polyvinyl fluoride(PVF), polyester, acrylic, or the like can be used.

As described above, the active matrix light-emitting device having thelight-emitting element of one embodiment of the present invention can beobtained.

Further, the light-emitting element of the present invention can be usedfor a passive matrix light-emitting device as well as the above activematrix light-emitting device. FIGS. 4A and 4B illustrate a perspectiveview and a cross-sectional view of a passive matrix light-emittingdevice using the light-emitting element of the present invention. FIG.4A is a perspective view of the light-emitting device, and FIG. 4B is across-sectional view taken along line X-Y of FIG. 4A.

In FIGS. 4A and 4B, an EL layer 504 is provided between a firstelectrode 502 and a second electrode 503 over a substrate 501. An endportion of the first electrode 502 is covered with an insulating layer505. In addition, a partition layer 506 is provided over the insulatinglayer 505. The sidewalls of the partition layer 506 are aslope so that adistance between both sidewalls is gradually narrowed toward the surfaceof the substrate. In other words, a cross section taken along thedirection of the short side of the partition layer 506 is trapezoidal,and the lower side (a side in contact with the insulating layer 505) isshorter than the upper side (a side not in contact with the insulatinglayer 505). By provision of the partition layer 506 in such a manner, adefect of the light-emitting element due to static electricity or thelike can be prevented.

Thus, the passive matrix light-emitting device having the light-emittingelement of one embodiment of the present invention can be obtained.

The light-emitting devices described in this embodiment (the activematrix light-emitting device and the passive matrix light-emittingdevice) are both formed using the light-emitting element of oneembodiment of the present invention, thereby having high emissionefficiency.

Note that in this embodiment, an appropriate combination of thestructures described in any other embodiment can be used.

Embodiment 5

Embodiment 5 will show electronic devices and lighting devices includingthe light-emitting device of one embodiment of the present inventiondescribed in Embodiment 4 as a part. Examples of the electronic devicesinclude cameras such as video cameras and digital cameras, goggle typedisplays, navigation systems, audio reproducing devices (e.g., car audiosystems and audio systems), computers, game machines, portableinformation terminals (e.g., mobile computers, cellular phones, portablegame machines, and electronic book readers), image reproducing devicesin which a recording medium is provided (specifically, devices that arecapable of reproducing recording media such as digital versatile discs(DVDs) and provided with a display device that can display an image),and the like. Specific examples of these electronic devices are shown inFIGS. 5A to 5D.

FIG. 5A illustrates a television set according to one embodiment of thepresent invention, which includes a housing 611, a supporting base 612,a display portion 613, speaker portions 614, video input terminals 615,and the like. In this television set, the light-emitting device of oneembodiment of the present invention can be applied to the displayportion 613. Since the light-emitting device of one embodiment of thepresent invention has high emission efficiency, a television set withlow power consumption can be obtained by application of thelight-emitting device of one embodiment of the present invention.

FIG. 5B illustrates a computer according to one embodiment of thepresent invention, which includes a main body 621, a housing 622, adisplay portion 623, a keyboard 624, an external connection port 625, apointing device 626, and the like. In this computer, the light-emittingdevice of the present invention can be applied to the display portion623. Since the light-emitting device of one embodiment of the presentinvention has high emission efficiency, a computer with low powerconsumption can be obtained by application of the light-emitting deviceof one embodiment of the present invention.

FIG. 5C shows a cellular phone of one embodiment of the presentinvention, which includes a main body 631, a housing 632, a displayportion 633, an audio input portion 634, an audio output portion 635,operation keys 636, an external connection port 637, an antenna 638, andthe like. In this cellular phone, the light-emitting device of thepresent invention can be applied to the display portion 633. Since thelight-emitting device of one embodiment of the present invention hashigh emission efficiency, a cellular phone having reduced powerconsumption can be obtained by application of the light-emitting deviceof one embodiment of the present invention.

FIG. 5D shows a camera of one embodiment of the present invention, whichincludes a main body 641, a display portion 642, a housing 643, anexternal connection port 644, a portion 645 for receiving signals from aremote control, an image receiving portion 646, a battery 647, an audioinput portion 648, operation keys 649, an eyepiece portion 650, and thelike. In this camera, the light-emitting device of one embodiment of thepresent invention can be applied to the display portion 642. Since thelight-emitting device of one embodiment of the present invention hashigh emission efficiency, a camera having reduced power consumption canbe obtained by application of the light-emitting device of oneembodiment of the present invention.

As thus described, application range of the light-emitting device of oneembodiment of the present invention is quite wide, and thislight-emitting device can be applied to electronic devices of a varietyof fields. With use of the light-emitting device of one embodiment ofthe present invention, an electronic device having reduced powerconsumption can be obtained.

Moreover, the light-emitting device of one embodiment of the presentinvention can be used as a lighting device. FIG. 6 illustrates anexample of a liquid crystal display device using the light-emittingdevice of one embodiment of the present invention as a backlight. Theliquid crystal display device illustrated in FIG. 6 includes a housing701, a liquid crystal layer 702, a backlight 703, and a housing 704. Theliquid crystal layer 702 is connected to a driver IC 705. Thelight-emitting device of one embodiment of the present invention is usedas the backlight 703, and a current is supplied through a terminal 706.

By using the light-emitting device of one embodiment of the presentinvention as a backlight of a liquid crystal display device as describedabove, a backlight having low power consumption can be obtained.Moreover, since the light-emitting device of one embodiment of thepresent invention is a lighting device for surface light emission andthe enlargement of the light-emitting device is possible, the backlightcan be made larger. Accordingly, a larger-area liquid crystal displaydevice having low power consumption can be obtained.

FIG. 7 illustrates an example in which the light-emitting device of oneembodiment of the present invention is used for a desk lamp which is alighting device. The desk lamp in FIG. 7 has a housing 801 and a lightsource 802, and the light-emitting device of one embodiment of thepresent invention is used as the light source 802. The light-emittingdevice of one embodiment of the present invention has the light-emittingelement having high emission efficiency and therefore can be used for adesk lamp having low power consumption.

FIG. 8 illustrates an example in which the light-emitting device of oneembodiment of the present invention is used for an indoor lightingdevice 901. Since the light-emitting device of an embodiment of thepresent invention can have a larger area, the light-emitting device ofan embodiment of the present invention can be used as a lighting systemhaving a large area. Further, the light-emitting device of oneembodiment of the present invention has the light-emitting elementhaving high emission efficiency and therefore can be used as a lightingdevice having low power consumption. In a room where the light-emittingdevice of one embodiment of the present invention is used as the indoorlighting device 901 as above, a television set 902 of one embodiment ofthe present invention as described referring to FIG. 5A can be installedso that pubic broadcasting and movies can be watched.

Note that the structure described of this embodiment can be implementedin combination with any of the structures described in other embodimentsas appropriate.

Example 1 Synthesis Example 1

This example will show a method of synthesizing4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2mDBFPPA-II) represented by the following Structural formula (100).

The method of synthesizing 2mDBFPPA-II is represented by SynthesisScheme (C-1), and reaction in the synthesis will be detailed below.

In a 100 mL three-neck flask were put 1.2 g (3.0 mmol) of2-bromo-9,10-diphenylanthracene, 0.87 g (3.0 mmol) of3-(dibenzofuran-4-yl)phenylboronic acid, and 0.23 g (0.75 mmol) oftri(ortho-tolyl)phosphine. The air in the flask was replaced withnitrogen. To this mixture were added 15 mL of toluene, 5.0 mL ofethanol, and 3.0 mL of a 2.0 mol/L aqueous solution of potassiumcarbonate. While the pressure was reduced, this mixture was stirred tobe degassed.

Then, 34 mg (0.15 mmol) of palladium(II) acetate was added to thismixture, and the mixture was stirred under a nitrogen stream at 80° C.for 4 hours. Then, the aqueous layer of this mixture was extracted withethyl acetate, and the extracted solution and the organic layer werecombined and washed with saturated brine. The organic layer was driedwith magnesium sulfate. Then, this mixture was gravity filtered. Theresulting filtrate was concentrated to give a solid, and the solid waspurified by silica gel column chromatography. The chromatography wascarried out using a mixed solvent having a 5:1 ratio of hexane totoluene as a developing solvent, whereby a solid was obtained.Recrystallization of the solid from a mixed solvent of toluene andhexane gave 1.4 g of a yellow powder in 79% yield, which was thesubstance to be produced.

By a train sublimation method, 1.4 g of the obtained yellow powderedsolid was purified. In the purification, the yellow powdered solid washeated at 270° C. under a pressure of 3.0 Pa with a flow rate of argongas of 4.0 mL/min. After the purification, 1.1 g of a yellow solid wasobtained in a yield of 81%, which was the substance to be produced.

A nuclear magnetic resonance (NMR) method identified this compound as4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2mDBFPPA-II), which was the substance to be produced.

¹H NMR data of the obtained compound are: ¹H NMR (CDCl₃, 300 MHz):δ=7.31-7.67 (m, 19H), 7.69-7.73 (m, 3H), 7.80-7.86 (m, 2H), 7.95 (dd,J₁=0.90 Hz, J₂=1.8 Hz, 1H), 7.98-8.01 (m, 2H), 8.07 (s, 1H).

FIGS. 9A and 9B show the ¹H NMR charts. Note that FIG. 9B is a chartshowing an enlarged part of FIG. 9A in the range of 7.2 to 8.2 ppm.

Thermogravimetry-differential thermal analysis (TG-DTA) of 2mDBFPPA-II,which was obtained, was performed. A high vacuum differential typedifferential thermal balance (manufactured by Bruker AXS K.K., TG/DTA2410SA) was used for the measurement. The measurement was carried outunder a nitrogen stream (a flow rate of 200 mL/min) and a normalpressure at a temperature rising rate of 10° C./min. The relationshipbetween weight and temperature (thermogravimetry) demonstrates that thetemperature at which the weight at the start of the measurement isreduced by 5% (5% weight loss temperature) is 418.0° C., which isindicative of high heat resistance.

Further, FIG. 10A shows an absorption spectrum of a toluene solution of2mDBFPPA-II, and FIG. 10B shows an emission spectrum thereof. FIG. 11Ashows an absorption spectrum of a thin film of 2mDBFPPA-II, and FIG. 11Bshows an emission spectrum thereof. The absorption spectrum was measuredusing an ultraviolet-visible spectrophotometer (V-550, produced by JASCOCorporation). The measurements were performed with samples prepared insuch a manner that the solution was put in a quartz cell while the thinfilm was obtained by evaporation onto a quartz substrate. The absorptionspectrum of the solution was obtained by subtracting the absorptionspectra of quartz and toluene from those of quartz and the solution, andthe absorption spectrum of the thin film was obtained by subtracting theabsorption spectrum of a quartz substrate from those of the quartzsubstrate and the thin film. In FIGS. 10A and 10B and FIGS. 11A and 11B,the horizontal axis represents wavelength (nm) and the vertical axisrepresents intensity (arbitrary unit). In the case of the toluenesolution, absorption was observed at around 406 nm, and the emissionwavelengths were 424 nm and 447 nm (excitation wavelength: 384 nm). Inthe case of the thin film, absorption was observed at around 246 nm, 289nm, 371 nm, 391 nm and 413 nm, and the emission wavelengths were 437 nmand 458 nm (excitation wavelength: 392 nm).

The HOMO level and the LUMO level of the thin film of 2mDBFPPA-II weremeasured. The value of the HOMO level was obtained by conversion of avalue of the ionization potential measured with a photoelectronspectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in theatmosphere into a negative value. The value of the LUMO level wasobtained in such a manner that the absorption edge, which was obtainedfrom Tauc plot with an assumption of direct transition using data on theabsorption spectrum of the thin film of 2mDBFPPA-II which is shown inFIG. 11A, was regarded as an optical energy gap and added to the valueof the HOMO level. As a result, the HOMO level and LUMO level of2mDBFPPA-II were found to be −5.66 eV and −2.79 eV, respectively.

The oxidation characteristic and reduction characteristic of 2mDBFPPA-IIwere measured. In the measurements of the oxidation and reductioncharacteristics, cyclic voltammetry (CV) measurement was employed, andan electrochemical analyzer (ALS model 600A, manufactured by BAS Inc.)was used.

As a solution used in the CV measurement, dehydratedN,N-dimethylformamide (DMF, product of Sigma-Aldrich Inc., 99.8%,catalog No. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,catalog No. T0836), which was a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Further, the object to be measured wasdissolved in the solvent such that the concentration thereof was 1mmol/L. A platinum electrode (manufactured by BAS Inc., PTE platinumelectrode) was used as a working electrode, a platinum electrode(manufactured by BAS Inc., Pt counter electrode for VC-3, (5 cm)) wasused as an auxiliary electrode, and an Ag/Ag⁺ electrode (manufactured byBAS Inc., RE-5 reference electrode for nonaqueous solvent) was used as areference electrode. Note that the measurement was conducted at roomtemperature. In addition, the scan rate at the CV measurement was set to0.1 V/s in all the measurement.

The reduction characteristic of 2mDBFPPA-II was examined by 100measurement cycles in which the potential of the working electrode withrespect to the reference electrode was scanned from −1.48 V to −2.27 Vand then from −2.27 V to −1.48 V in each cycle. Similarly, the oxidationcharacteristic of 2mDBFPPA-II was evaluated by 100 measurement cycles inwhich the potential of the working electrode with respect to thereference electrode was scanned from 0.18 V to 1.02 V and then from 1.02V to 0.18 V in each cycle.

According to the measurement results, a peak current corresponding tooxidation at around 0.89 V (vs. Ag/Ag⁺) and a peak current correspondingto reduction at around −2.16 V (vs. Ag/Ag⁺) were observed. FIG. 12 showsa graph of the results.

Even after as many as 100 scan cycles, 2mDBFPPA-II showed no significantchange in the peak position of the CV curves representing oxidation andreduction and kept the peak intensity at 76% of the initial intensity onthe oxidation side and at 90% on the reduction side. Thus, it isunderstood that 2mDBFPPA-II is relatively stable, when subjected torepetitions of oxidation from a neutral state to an oxidized state andreduction from the oxidized state to the neutral state or repetitions ofreduction from a neutral state to a reduced state and oxidation from thereduced state to the neutral state.

Example 2 Synthesis Example 2

This example will show a method of synthesizing4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene (abbreviation:2mDBTPPA-II) represented by the following Structural formula (300).

The method of synthesizing 2mDBTPPA-II is represented by SynthesisScheme (D-1), and reaction in the synthesis will be detailed below.

In a 100 mL three-neck flask were put 1.6 g (4.0 mmol) of2-bromo-9,10-diphenylanthracene, 1.2 g (4.0 mmol) of3-(dibenzothiophen-4-yl)phenylboronic acid, and 0.30 g (1.0 mmol) oftri(ortho-tolyl)phosphine. The air in the flask was replaced withnitrogen. To this mixture were added 25 mL of toluene, 5.0 mL ofethanol, and 5.0 mL of a 2.0 mol/L aqueous solution of potassiumcarbonate. While the pressure was reduced, this mixture was stirred tobe degassed.

Then, 45 mg (0.20 mmol) of palladium(II) acetate was added to thismixture, and the mixture was stirred under a nitrogen stream at 80° C.for 5 hours. Then, the aqueous layer of this mixture was extracted withtoluene, and the toluene solution and the organic layer were combinedand washed with saturated brine. The organic layer was dried withmagnesium sulfate. Then, this mixture was gravity filtered. Theresulting filtrate was concentrated to give an oily substance. Theobtained oily substance was purified by silica gel columnchromatography. The chromatography was carried out using a mixed solventhaving a 5:1 ratio of hexane to toluene as a developing solvent, wherebya solid was obtained. Recrystallization of the solid from a mixedsolvent of toluene and hexane gave 1.6 g of a yellow powder in 70%yield, which was the substance to be produced.

By a train sublimation method, 1.6 g of the obtained yellow powderedsolid was purified. In the purification, the yellow powdered solid washeated at 290° C. under a pressure of 3.0 Pa with a flow rate of argongas of 4.0 mL/min. After the purification, 1.4 g of a yellow solid wasobtained in a yield of 87%, which was the substance to be produced.

A nuclear magnetic resonance (NMR) method identified this compound as4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene (abbreviation:2mDBTPPA-II), which was the substance to be produced.

¹H NMR data of the obtained compound are: ¹H NMR (CDCl₃, 300 MHz):δ=7.33 (q, J₁=3.3 Hz, 2H), 7.46-7.73 (m, 20H), 7.80-7.87 (m, 2H), 7.99(st, J₁=1.8 Hz, 1H), 8.03 (sd, J₁=1.5 Hz, 1H), 8.14-8.20 (m, 2H).

FIGS. 13A and 13B show the ¹H NMR charts. Note that FIG. 13B is a chartshowing an enlarged part of FIG. 13A in the range of 7.2 to 8.3 ppm.

Thermogravimetry-differential thermal analysis (TG-DTA) of 2mDBTPPA-II,which was obtained, was performed. A high vacuum differential typedifferential thermal balance (manufactured by Bruker AXS K.K., TG/DTA2410SA) was used for the measurement. The measurement was carried outunder a nitrogen stream (a flow rate of 200 mL/min) and a normalpressure at a temperature rising rate of 10° C./min. The relationshipbetween weight and temperature (thermogravimetry) demonstrates that the5% weight loss temperature is 441.1° C., which is indicative of highheat resistance.

Further, FIG. 14A shows an absorption spectrum of a toluene solution of2mDBTPPA-II, and FIG. 14B shows an emission spectrum thereof. FIG. 15Ashows an absorption spectrum of a thin film of 2mDBTPPA-II, and FIG. 15Bshows an emission spectrum thereof. The absorption spectrum was measuredusing an ultraviolet-visible spectrophotometer (V-550, produced by JASCOCorporation). The measurements were performed with samples prepared insuch a manner that the solution was put in a quartz cell while the thinfilm was obtained by evaporation onto a quartz substrate. The absorptionspectrum of the solution was obtained by subtracting the absorptionspectra of quartz and toluene from those of quartz and the solution, andthe absorption spectrum of the thin film was obtained by subtracting theabsorption spectrum of a quartz substrate from those of the quartzsubstrate and the thin film. In FIGS. 14A and 14B and FIGS. 15A and 15B,the horizontal axis represents wavelength (nm) and the vertical axisrepresents intensity (arbitrary unit). In the case of the toluenesolution, absorption was observed at around 407 nm, and the emissionwavelengths were 423 nm and 446 nm (excitation wavelength: 385 nm). Inthe case of the thin film, absorption was observed at around 244 nm, 293nm, 371 nm, 392 nm and 414 nm, and the emission wavelengths were 437 nmand 459 nm (excitation wavelength: 391 nm).

The HOMO level and the LUMO level of the thin film of 2mDBTPPA-II weremeasured. The value of the HOMO level was obtained by conversion of avalue of the ionization potential measured with a photoelectronspectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in theatmosphere into a negative value. The value of the LUMO level wasobtained in such a manner that the absorption edge, which was obtainedfrom Tauc plot with an assumption of direct transition using data on theabsorption spectrum of the thin film of 2mDBTPPA-II which is shown inFIG. 15A, was regarded as an optical energy gap and added to the valueof the HOMO level. As a result, the HOMO level and LUMO level of2mDBTPPA-II were found to be −5.71 eV and −2.85 eV, respectively.

The oxidation characteristic and reduction characteristic of 2mDBTPPA-Hwere measured. In the measurements of the oxidation and reductioncharacteristics, cyclic voltammetry (CV) measurement was employed, andan electrochemical analyzer (ALS model 600A, manufactured by BAS Inc.)was used.

As a solution used in the CV measurement, dehydratedN,N-dimethylformamide (DMF, product of Sigma-Aldrich Inc., 99.8%,catalog No. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,catalog No. T0836), which was a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Further, the object to be measured wasdissolved in the solvent such that the concentration thereof was 1mmol/L. A platinum electrode (manufactured by BAS Inc., PTE platinumelectrode) was used as a working electrode, a platinum electrode(manufactured by BAS Inc., Pt counter electrode for VC-3, (5 cm)) wasused as an auxiliary electrode, and an Ag/Ag⁺ electrode (manufactured byBAS Inc., RE-5 reference electrode for nonaqueous solvent) was used as areference electrode. Note that the measurement was conducted at roomtemperature. In addition, the scan rate at the CV measurement was set to0.1 V/s in all the measurement.

The reduction characteristic of 2mDBTPPA-II was examined by 100measurement cycles in which the potential of the working electrode withrespect to the reference electrode was scanned from −1.46 V to −2.25 Vand then from −2.25 V to −1.46 V in each cycle. Similarly, the oxidationcharacteristic of 2mDBTPPA-II was evaluated by 100 measurement cycles inwhich the potential of the working electrode with respect to thereference electrode was scanned from 0.32 V to 1.00 V and then from 1.00V to 0.32 V in each cycle.

According to the measurement results, a peak current corresponding tooxidation at around 0.88 V (vs. Ag/Ag⁺) and a peak current correspondingto reduction at around −2.16 V (vs. Ag/Ag⁺) were observed. FIG. 16 showsa graph of the results.

Even after as many as 100 scan cycles, 2mDBTPPA-II showed no significantchange in the peak position of the CV curves representing oxidation andreduction and kept the peak intensity at 76% of the initial intensity onthe oxidation side and at 90% on the reduction side. Thus, it isunderstood that 2mDBTPPA-II is relatively stable, when subjected torepetitions of oxidation from a neutral state to an oxidized state andreduction from the oxidized state to the neutral state or repetitions ofreduction from a neutral state to a reduced state and oxidation from thereduced state to the neutral state.

Example 3

In this example, a light-emitting element of one embodiment of thepresent invention will be described referring to FIG. 37A. Chemicalformulae of materials used in this example are shown below.

Methods of fabricating Light-emitting Element 1 of this example andReference Light-emitting Element 1 will now be described.

(Light-Emitting Element 1)

First, indium tin oxide containing silicon oxide (ITSO) was deposited bya sputtering method on a glass substrate 1100, whereby a first electrode1101 was formed. Its thickness was 110 nm and the electrode area was 2mm×2 mm. Here, the first electrode 1101 is an electrode that functionsas an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, a surface of the substrate was washed with water, bakedat 200° C. for one hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a substrate holder in a vacuumevaporation apparatus so that a surface of the substrate 1100 on whichthe first electrode 1101 was formed faced downward. The pressure in thevacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. Then, by anevaporation method using resistance heating,9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA)which is a substance having a high hole-transport property andmolybdenum(VI) oxide which is an acceptor substance were co-evaporatedto form a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was 50 nm, and the weightratio of CzPA to molybdenum(VI) oxide was controlled to be 4:2(=CzPA:molybdenum(VI) oxide). Note that the co-evaporation method refersto an evaporation method in which evaporation is carried out from aplurality of evaporation sources at the same time in one treatmentchamber.

Next, 4-phenyl-4′-[4-(9-phenylfluoren-9-yl)phenyl]triphenylamine(abbreviation: BPAFLBi) was deposited to a thickness of 10 nm over thehole-injection layer 1111, whereby a hole-transport layer 1112 wasformed.

Furthermore, 2mDBFPPA-II synthesized in Example 1 and4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA) were co-evaporated to form a light-emitting layer1113 over the hole-transport layer. The weight ratio of 2mDBFPPA-II toPCBAPA was adjusted to 1:0.1 (=2mDBFPPA-II:PCBAPA). The thickness of thelight-emitting layer 1113 was set to 30 nm.

Then, over the light-emitting layer 1113, a 10 nm thick layer oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq) and, a 15 nmthick layer of bathophenanthroline (abbreviation: BPhen) were depositedon the Alq layer, whereby an electron-transport layer 1114 including Alqand BPhen was obtained.

Further, a 1 nm thick film of lithium fluoride (LiF) was formed over theelectron-transport layer 1114 by evaporation, whereby anelectron-injection layer 1115 was formed.

Lastly, a 200 nm thick film of aluminum was formed by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 1 of this example was fabricated.

Note that, in the above evaporation process, evaporation was allperformed by a resistance heating method.

(Reference Light-Emitting Element 1)

The light-emitting layer 1113 of Reference Light-emitting Element 1 wasformed by co-evaporation of4-[4-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2DBFPPA-II) and PCBAPA, instead of the material used for Light-emittingElement 1. The weight ratio of 2DBFPPA-II and PCBAPA was adjusted to1:0.1 (=2DBFPPA-II:PCBAPA). The thickness of the light-emitting layer1113 was set to 30 nm. The layers other than the light-emitting layer1113 were formed in the same manner as Light-emitting Element 1.

Table 1 shows element structures of Light-emitting Element 1 andReference Light-emitting Element 1 formed as described above.

TABLE 1 Hole- Hole- Light- Electron- Electron- First injection transportemitting transport injection Second electrode layer layer layer layerlayer electrode Light-emitting ITSO CzPA:MoOx BPAFLBi 2mDBFPPA-II:PCBAPAAlq BPhen LiF Al Element 1 110 nm (=4:2) 10 nm (=1:0.1) 10 nm 15 nm 1 nm200 nm 50 nm 30 nm Reference ITSO CzPA:MoOx BPAFLBi 2DBFPPA-II:PCBAPAAlq BPhen LiF Al Light-emitting 110 nm (=4:2) 10 nm (=1:0.1) 10 nm 15 nm1 nm 200 nm Element 1 50 nm 30 nm

Light-emitting Element 1 and Reference Light-emitting Element 1 weresealed in a glove box containing a nitrogen atmosphere so as not to beexposed to air. Then, operation characteristics of the elements weremeasured. Note that the measurement was carried out at room temperature(in the atmosphere kept at 25° C.).

FIG. 17 shows luminance vs. current density characteristics ofLight-emitting Element 1 and Reference Light-emitting Element 1. In FIG.17, the horizontal axis represents luminance (cd/m²) and the verticalaxis represents current efficiency (cd/A). FIG. 18 shows the voltage vs.luminance characteristics. In FIG. 18, the horizontal axis representsapplied voltage (V) and the vertical axis represents luminance (cd/m²).FIG. 19 shows the luminance vs. external quantum efficiencycharacteristics. In FIG. 19, the horizontal axis represents luminance(cd/m²) and the vertical axis represents external quantum efficiency(%). FIG. 20 shows the emission spectra with a current supply of 1 mA.In FIG. 20, the horizontal axis represents wavelength (nm) and thevertical axis represents intensity (arbitrary unit). Further, Table 2shows the voltage (V), current density (mA/cm²), CIE chromaticitycoordinates (x, y), current efficiency (cd/A), and external quantumefficiency (%) of each light-emitting element at a luminance of around1000 cd/m².

TABLE 2 External Current Current quantum Voltage density ChromaticityChromaticity Luminance efficiency efficiency (V) (mA/cm²) x y (cd/m²)(cd/A) (%) Light-emitting 4.6 17 0.17 0.21 980 5.7 3.8 Element 1Reference 4.6 19 0.18 0.24 960 5.2 3.1 Light-emitting Element 1

As seen from FIG. 20 and the CIE chromaticity coordinates in Table 2,blue light emission is shown by Light-emitting Element 1 and ReferenceLight-emitting Element 1, which were formed. FIG. 17, FIG. 18, FIG. 19,and Table 2 reveal that Light-emitting Element 1 exhibits betterchromaticity and higher current efficiency and external quantumefficiency than those of Reference Light-emitting Element 1.

As described above, 2mDBFPPA-II produced in Example 1 was used as thehost material of the light-emitting layer, whereby the light-emittingelement achieved good chromaticity and high emission efficiency.

Next, Light-emitting Element 1 and Reference Light-emitting Element 1were subjected to reliability tests. Results of the reliability testsare shown in FIG. 21. In FIG. 21, the vertical axis representsnormalized luminance (%) with an initial luminance of 100%, and thehorizontal axis represents driving time (h) of the elements. In thereliability tests, Light-emitting Element 1 of this example andReference Light-emitting Element 1 were driven under the conditionswhere the current density was constant and the initial luminance was1000 cd/m². FIG. 21 shows that Light-emitting Element 1 and ReferenceLight-emitting Element 1 kept 86% of the initial luminance after thedriving for 410 hours. Thus, Light-emitting Element 1 is equal inreliability to Reference Light-emitting Element 1 as well as has betterchromaticity and higher emission efficiency than those of ReferenceLight-emitting Element 1.

Light-emitting Element 1 exhibited better chromaticity and higheremission efficiency than those of Reference Light-emitting Element 1.The difference in host material structure between the light-emittinglayers of Light-emitting Element 1 and Reference Light-emitting Element1 is that the 2-position of an anthracene skeleton and the 4-position ofa dibenzofuran skeleton in a dibenzofuran derivative which is the hostmaterial are bonded through a phenylene group at the para-position inReference Light-emitting Element 1 while bonded through a phenylenegroup at the meta-position in Light-emitting Element 1. Whether whatlies between the 2-position of the anthracene skeleton and the4-position of the dibenzofuran skeleton is the phenylene group at thepara-position or that at the meta-position makes a difference inemission efficiency between Light-emitting Element 1 and ReferenceLight-emitting Element 1. This reveals that the dibenzofuran derivativeof one embodiment of the present invention is effective in realizinghigh emission efficiency, in respect of its structure where the2-position of the anthracene skeleton and the 4-position of thedibenzofuran skeleton are bonded through the phenylene group at themeta-position. Further, it is understood that, by using the dibenzofuranderivative of one embodiment of the present invention in alight-emitting element, the light-emitting element can provide goodchromaticity and high emission efficiency.

Example 4

In this example, a light-emitting element of one embodiment of thepresent invention will be described referring to FIG. 37B.

Methods of fabricating Light-emitting Element 2 of this example andReference Light-emitting Element 2 will now be described. Materials usedin the present example are the same as those used in Example 3, andtheir chemical formulae are omitted here.

(Light-Emitting Element 2)

First, ITSO was deposited over the glass substrate 1100 by a sputteringmethod, whereby the first electrode 1101 was formed. Its thickness was110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode1101 is an electrode that functions as an anode of the light-emittingelement.

Next, as pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water, baked at200° C. for one hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate wascooled down for about 30 minutes.

Next, the substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that a surface of the substrate on which thefirst electrode was formed faced downward. The pressure in the vacuumevaporation apparatus was reduced to about 10⁻⁴ Pa. Then, CzPA which isa substance having a high hole-transport property and molybdenum(VI)oxide which is an acceptor substance were co-evaporated to form thehole-injection layer 1111 over the first electrode. The thickness of thehole-injection layer 1111 was 50 nm, and the weight ratio of CzPA tomolybdenum(VI) oxide was controlled to be 4:2 (=CzPA:molybdenum(VI)oxide).

Next, BPAFLBi was deposited to a thickness of 10 nm over thehole-injection layer 1111, whereby the hole-transport layer 1112 wasformed.

Further, PCBAPA was deposited to a thickness of 25 nm over thehole-transport layer 1112, whereby a first light-emitting layer 1113 awas formed. Then, 2mDBFPPA-II synthesized in Example 1 and PCBAPA wereco-evaporated to form a second light-emitting layer 1113 b over thefirst light-emitting layer 1113 a. The weight ratio of 2mDBFPPA-II toPCBAPA was adjusted to 1:0.1 (=2mDBFPPA-II:PCBAPA). The thickness of thesecond light-emitting layer 1113 b was set to 30 nm.

Then, on the second light-emitting layer 1113 b, a 10 nm thick layer ofAlq and, a 15 nm thick layer of BPhen were deposited on the Alq layer,whereby the electron-transport layer 1114 including Alq and BPhen wasobtained.

Further, a 1 nm thick film of LiF was formed over the electron-transportlayer 1114 by evaporation, whereby the electron-injection layer 1115 wasformed.

Lastly, a 200 nm thick film of aluminum was formed by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 2 of this example was fabricated.

Note that, in the above evaporation process, evaporation was allperformed by a resistance heating method.

(Reference Light-Emitting Element 2)

The second light-emitting layer 1113 b of Reference Light-emittingElement 2 was formed by co-evaporation of 2DBFPPA-II and PCBAPA, insteadof the material used for Light-emitting Element 2. The weight ratio of2DBFPPA-II and PCBAPA was adjusted to 1:0.1 (=2DBFPPA-II:PCBAPA). Thethickness of the second light-emitting layer 1113 b was set to 30 nm.The layers other than the second light-emitting layer 1113 b were formedin the same manner as Light-emitting Element 2.

Table 3 shows element structures of Light-emitting Element 2 andReference Light-emitting Element 2 formed as described above.

TABLE 3 Hole- Hole- First Second Electron- Electron- First injectiontransport light-emitting light-emitting transport injection Secondelectrode layer layer layer layer layer layer electrode Light-emittingITSO CzPA:MoOx BPAFLBi PCBAPA 2mDBFPPA-II:PCBAPA Alq BPhen LiF AlElement 2 110 nm (=4:2) 10 nm 25 nm (=1:0.1) 10 nm 15 nm 1 nm 200 nm 50nm 30 nm Reference ITSO CzPA:MoOx BPAFLBi PCBAPA 2DBFPPA-II:PCBAPA AlqBPhen LiF Al Light-emitting 110 nm (=4:2) 10 nm 25 nm (=1:0.1) 10 nm 15nm 1 nm 200 nm Element 2 50 nm 30 nm

Light-emitting Element 2 and Reference Light-emitting Element 2 weresealed in a glove box containing a nitrogen atmosphere so as not to beexposed to air. Then, operation characteristics of the elements weremeasured. Note that the measurement was carried out at room temperature(in the atmosphere kept at 25° C.).

FIG. 22 shows luminance vs. current density characteristics ofLight-emitting Element 2 and Reference Light-emitting Element 2. In FIG.22, the horizontal axis represents luminance (cd/m²) and the verticalaxis represents current efficiency (cd/A). FIG. 23 shows the voltage vs.luminance characteristics. In FIG. 23, the horizontal axis representsapplied voltage (V) and the vertical axis represents luminance (cd/m²).FIG. 24 shows the luminance vs. external quantum efficiencycharacteristics. In FIG. 24, the horizontal axis represents luminance(cd/m²) and the vertical axis represents external quantum efficiency(%). FIG. 25 shows the emission spectra with a current supply of 1 mA.In FIG. 25, the horizontal axis represents wavelength (nm) and thevertical axis represents intensity (arbitrary unit). Further, Table 4shows the voltage (V), current density (mA/cm²), CIE chromaticitycoordinates (x, y), current efficiency (cd/A), and external quantumefficiency (%) of each light-emitting element at a luminance of around1000 cd/m².

TABLE 4 External Current Current quantum Voltage density ChromaticityChromaticity Luminance efficiency efficiency (V) (mA/cm²) x y (cd/m²)(cd/A) (%) Light-emitting 5.0 14 0.16 0.25 880 6.4 4.0 Element 2Reference 5.2 19 0.17 0.26 1100 5.7 3.4 Light-emitting Element 2

As seen from FIG. 25 and the CIE chromaticity coordinates in Table 4,blue light emission is shown by Light-emitting Element 2 and ReferenceLight-emitting Element 2, which were formed. FIG. 22, FIG. 23, FIG. 24,and Table 4 reveal that Light-emitting Element 2 exhibits higher currentefficiency and external quantum efficiency than those of ReferenceLight-emitting Element 2.

As described above, 2mDBFPPA-II produced in Example 1 was used as thehost material of the light-emitting layer, whereby the light-emittingelement achieved high emission efficiency when the light-emitting layerhad a two-layer structure as well.

Next, Light-emitting Element 2 and Reference Light-emitting Element 2were subjected to reliability tests. Results of the reliability testsare shown in FIG. 26. In FIG. 26, the vertical axis representsnormalized luminance (%) with an initial luminance of 100%, and thehorizontal axis represents driving time (h) of the elements. In thereliability tests, Light-emitting Element 2 of this example andReference Light-emitting Element 2 were driven under the conditionswhere the current density was constant and the initial luminance was1000 cd/m². FIG. 26 shows that Light-emitting Element 2 and ReferenceLight-emitting Element 2 kept 91% of the initial luminance after thedriving for 170 hours. Thus, Light-emitting Element 2 is equal inreliability to Reference Light-emitting Element 2 as well as higheremission efficiency than those of Reference Light-emitting Element 2.Furthermore, the results of the reliability tests demonstrate that thelight-emitting element to which one embodiment of the present inventionis applied is effective in realizing a light-emitting element having along lifetime.

Light-emitting Element 2 exhibited better chromaticity and higheremission efficiency than those of Reference Light-emitting Element 2.The difference in host material structure between the light-emittinglayers of Light-emitting Element 2 and Reference Light-emitting Element2 is that the 2-position of an anthracene skeleton and the 4-position ofa dibenzofuran skeleton in a dibenzofuran derivative which is the hostmaterial are bonded through a phenylene group at the para-position inReference Light-emitting Element 2 while bonded through a phenylenegroup at the meta-position in Light-emitting Element 2. Whether whatlies between the 2-position of the anthracene skeleton and the4-position of the dibenzofuran skeleton is the phenylene group at thepara-position or that at the meta-position makes a difference inemission efficiency between Light-emitting Element 2 and ReferenceLight-emitting Element 2. This reveals that the dibenzofuran derivativeof one embodiment of the present invention is effective in realizinghigh emission efficiency, in respect of its structure where the2-position of the anthracene skeleton and the 4-position of thedibenzofuran skeleton are bonded through the phenylene group at themeta-position. Further, it is understood that, by using the dibenzofuranderivative of one embodiment of the present invention in alight-emitting element, the light-emitting element can provide goodchromaticity and high emission efficiency.

Example 5

In this example, a light-emitting element of one embodiment of thepresent invention will be described referring to FIG. 37A. Chemicalformulae of materials used in this example are shown below. Note thatthe materials the chemical formulae of which are described above will beomitted.

A method of fabricating Light-emitting Element 3 of this example willnow be described.

(Light-Emitting Element 3)

First, ITSO was deposited over the glass substrate 1100 by a sputteringmethod, whereby the first electrode 1101 was formed. Its thickness was110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode1101 is an electrode that functions as an anode of the light-emittingelement.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, a surface of the substrate was washed with water, bakedat 200° C. for one hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a substrate holder in a vacuumevaporation apparatus so that a surface of the substrate 1100 on whichthe first electrode 1101 was formed faced downward. The pressure in thevacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. Then, by anevaporation method using resistance heating, CzPA which is a substancehaving a high hole-transport property and molybdenum(VI) oxide which isan acceptor substance were co-evaporated to form the hole-injectionlayer 1111 over the first electrode 1101. The thickness of thehole-injection layer 1111 was 50 nm, and the weight ratio of CzPA tomolybdenum(VI) oxide was controlled to be 4:2 (=CzPA:molybdenum(VI)oxide). Note that the co-evaporation method refers to an evaporationmethod in which evaporation is carried out from a plurality ofevaporation sources at the same time in one treatment chamber.

Next, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was deposited to a thickness of 10 nm over the hole-injectionlayer 1111, whereby the hole-transport layer 1112 was formed.

Further, 2mDBTPPA-II synthesized in Example 2 and PCBAPA wereco-evaporated to form the light-emitting layer 1113 over thehole-transport layer 1112. The weight ratio of 2mDBTPPA-II to PCBAPA wasadjusted to 1:0.1 (=2mDBTPPA-II:PCBAPA). The thickness of thelight-emitting layer 1113 was set to 30 nm.

Then, over the light-emitting layer 1113, a 10 nm thick layer of Alqand, a 15 nm thick layer of BPhen were deposited on the Alq layer,whereby the electron-transport layer 1114 including Alq and BPhen wasobtained.

Further, a 1 nm thick film of LiF was formed over the electron-transportlayer 1114 by evaporation, whereby the electron-injection layer 1115 wasformed.

Lastly, a 200 nm thick film of aluminum was formed by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 3 of this example was fabricated.

Note that, in the above evaporation process, evaporation was allperformed by a resistance heating method.

Table 5 shows element structures of Light-emitting Element 3 formed asdescribed above.

TABLE 5 Hole- Hole- Light- Electron- Electron- First injection transportemitting transport injection Second electrode layer layer layer layerlayer electrode Light-emitting ITSO CzPA:MoOx BPAFLP 2mDBTPPA-II:PCBAPAAlq BPhen LiF Al Element 3 110 nm (=4:2) 10 nm (=1:0.1) 10 nm 15 nm 1 nm200 nm 50 nm 30 nm

Light-emitting Element 3 was sealed in a glove box containing a nitrogenatmosphere so as not to be exposed to air. Then, operationcharacteristics of the element were measured. Note that the measurementwas carried out at room temperature (in the atmosphere kept at 25° C.).

FIG. 27 shows luminance vs. current density characteristics ofLight-emitting Element 3. In FIG. 27, the horizontal axis representsluminance (cd/m²) and the vertical axis represents current efficiency(cd/A). FIG. 28 shows the voltage vs. luminance characteristics. In FIG.28, the horizontal axis represents applied voltage (V) and the verticalaxis represents luminance (cd/m²). FIG. 29 shows the luminance vs.external quantum efficiency characteristics. In FIG. 29, the horizontalaxis represents luminance (cd/m²) and the vertical axis representsexternal quantum efficiency (%). FIG. 30 shows the emission spectra witha current supply of 1 mA. In FIG. 30, the horizontal axis representswavelength (nm) and the vertical axis represents intensity (arbitraryunit). Further, Table 6 shows the voltage (V), current density (mA/cm²),CIE chromaticity coordinates (x, y), current efficiency (cd/A), andexternal quantum efficiency (%) of the light-emitting element at aluminance of around 810 cd/m².

TABLE 6 External Current Current quantum Voltage density ChromaticityChromaticity Luminance efficiency efficiency (V) (mA/cm²) x y (cd/m²)(cd/A) (%) Light-emitting 4.0 12 0.17 0.24 810 6.8 4.1 Element 3

As seen from FIG. 30 and the CIE chromaticity coordinates in Table 6,blue light emission is shown by Light-emitting Element 3, which wasformed. FIG. 27, FIG. 28, FIG. 29, and Table 6 reveal thatLight-emitting Element 3 exhibits good chromaticity, high currentefficiency, and high external quantum efficiency.

As described above, 2mDBTPPA-II produced in Example 2 was used as thehost material of the light-emitting layer, whereby the light-emittingelement achieved high emission efficiency.

Next, Light-emitting Element 3 was subjected to reliability tests.Results of the reliability tests are shown in FIG. 31. In FIG. 31, thevertical axis represents normalized luminance (%) with an initialluminance of 100%, and the horizontal axis represents driving time (h)of the elements. In the reliability tests, Light-emitting Element 3 ofthis example was driven under the conditions where the current densitywas constant and the initial luminance was 1000 cd/m². FIG. 31 showsthat Light-emitting Element 3 kept 80% of the initial luminance afterthe driving for 220 hours. Thus, Light-emitting Element 3 shows highreliability. Furthermore, the results of the reliability testsdemonstrate that the light-emitting element to which one embodiment ofthe present invention is applied is effective in realizing alight-emitting element having a long lifetime.

Example 6

In this example, a light-emitting element of one embodiment of thepresent invention will be described referring to FIG. 37B.

A method of fabricating Light-emitting Element 4 of this example willnow be described. Materials used in the present example are the same asthose used in Example 5, and their chemical formulae are omitted here.

(Light-Emitting Element 4)

First, ITSO was deposited over the glass substrate 1100 by a sputteringmethod, whereby the first electrode 1101 was formed. Its thickness was110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode1101 is an electrode that functions as an anode of the light-emittingelement.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, a surface of the substrate was washed with water, bakedat 200° C. for one hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a substrate holder in a vacuumevaporation apparatus so that a surface of the substrate 1100 on whichthe first electrode 1101 was formed faced downward. The pressure in thevacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. Then, by anevaporation method using resistance heating, CzPA which is a substancehaving a high hole-transport property and molybdenum(VI) oxide which isan acceptor substance were co-evaporated to form the hole-injectionlayer 1111 over the first electrode 1101. The thickness of thehole-injection layer 1111 was 50 nm, and the weight ratio of CzPA tomolybdenum(VI) oxide was controlled to be 4:2 (=CzPA:molybdenum(VI)oxide). Note that the co-evaporation method refers to an evaporationmethod in which evaporation is carried out from a plurality ofevaporation sources at the same time in one treatment chamber.

Next, BPAFLP was deposited to a thickness of 10 nm over thehole-injection layer 1111, whereby the hole-transport layer 1112 wasformed.

Further, PCBAPA was deposited to a thickness of 25 nm over thehole-transport layer 1112, whereby the first light-emitting layer 1113 awas formed. Then, 2mDBTPPA-II synthesized in Example 2 and PCBAPA wereco-evaporated to form the second light-emitting layer 1113 b over thefirst light-emitting layer 1113 a. The weight ratio of 2mDBTPPA-II toPCBAPA was adjusted to 1:0.1 (=2mDBTPPA-II:PCBAPA). The thickness of thesecond light-emitting layer 1113 b was set to 30 nm.

Then, on the second light-emitting layer 1113 b, a 10 nm thick layer ofAlq and, a 15 nm thick layer of BPhen were deposited on the Alq layer,whereby the electron-transport layer 1114 including Alq and BPhen wasobtained.

Further, a 1 nm thick film of LiF was formed over the electron-transportlayer 1114 by evaporation, whereby the electron-injection layer 1115 wasformed.

Lastly, a 200 nm thick film of aluminum was formed by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 4 of this example was fabricated.

Note that, in the above evaporation process, evaporation was allperformed by a resistance heating method.

Table 7 shows element structures of Light-emitting Element 4 formed asdescribed above.

TABLE 7 Hole- Hole- First Second Electron- Electron- First injectiontransport light-emitting light-emitting transport injection Secondelectrode layer layer layer layer layer layer electrode Light-emittingITSO CzPA:MoOx BPAFLP PCBAPA 2mDBTPPA-II:PCBAPA Alq BPhen LiF Al Element4 110 nm (=4:2) 10 nm 25 nm (=1:0.1) 10 nm 15 nm 1 nm 200 nm 50 nm 30 nm

Light-emitting Element 4 was sealed in a glove box containing a nitrogenatmosphere so as not to be exposed to air. Then, operationcharacteristics of the element were measured. Note that the measurementwas carried out at room temperature (in the atmosphere kept at 25° C.).

FIG. 32 shows luminance vs. current density characteristics ofLight-emitting Element 4. In FIG. 32, the horizontal axis representsluminance (cd/m²) and the vertical axis represents current efficiency(cd/A). FIG. 33 shows the voltage vs. luminance characteristics. In FIG.33, the horizontal axis represents applied voltage (V) and the verticalaxis represents luminance (cd/m²). FIG. 34 shows the luminance vs.external quantum efficiency characteristics. In FIG. 34, the horizontalaxis represents luminance (cd/m²) and the vertical axis representsexternal quantum efficiency (%). FIG. 35 shows the emission spectra witha current supply of 1 mA. In FIG. 35, the horizontal axis representswavelength (nm) and the vertical axis represents intensity (arbitraryunit). Further, Table 8 shows the voltage (V), current density (mA/cm²),CIE chromaticity coordinates (x, y), current efficiency (cd/A), andexternal quantum efficiency (%) of the Light-emitting Element 4 at aluminance of around 1100 cd/m².

TABLE 8 External Current Current quantum Voltage density ChromaticityChromaticity Luminance efficiency efficiency (V) (mA/cm²) x y (cd/m²)(cd/A) (%) Light-emitting 4.4 16 0.16 0.23 1100 7.1 4.4 Element 4

As seen from in FIG. 35 and the CIE chromaticity coordinates in Table 8,blue light emission is shown by Light-emitting Element 4, which wasformed. FIG. 32, FIG. 33, FIG. 34, and Table 8 reveal thatLight-emitting Element 4 exhibits good chromaticity, high currentefficiency, and high external quantum efficiency.

As described above, 2mDBTPPA-II produced in Example 2 was used as thehost material of the light-emitting layer, whereby the light-emittingelement achieved high emission efficiency when the light-emitting layerhad a two-layer structure as well.

Next, Light-emitting Element 4 was subjected to reliability tests.Results of the reliability tests are shown in FIG. 36. In FIG. 36, thevertical axis represents normalized luminance (%) with an initialluminance of 100%, and the horizontal axis represents driving time (h)of the elements. In the reliability tests, Light-emitting Element 4 ofthis example was driven under the conditions where the current densitywas constant and the initial luminance was 1000 cd/m². FIG. 36 showsthat Light-emitting Element 4 kept 80% of the initial luminance afterthe driving for 170 hours. Thus, Light-emitting Element 4 shows highreliability. Furthermore, the results of the reliability testsdemonstrate that the light-emitting element to which one embodiment ofthe present invention is applied is effective in realizing alight-emitting element having a long lifetime.

Example 7 Synthesis Example 3

This example will show a method of synthesizing4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran(abbreviation: 2mDBFPPA-III) represented by Structural formula (147)described in Embodiment 1.

Step 1: Synthesis of 2,8-dibromodibenzofuran

The synthesis scheme of Step 1 is shown in (E-1).

In a 500 mL three-neck flask were put 8.4 g (50 mmol) of dibenzofuranand 100 mL of carbon tetrachloride. A solution prepared by dissolving 17g (110 mmol) of bromine in 50 mL of chloroform was dripped through adropping funnel into the three-neck flask over about 20 minutes. Then,this solution was stirred at room temperature for 7 days. After that,this solution was washed with a saturated solution of sodium hydrogencarbonate, an aqueous solution of sodium thiosulfate and saturatedbrine. The organic layer was dried with magnesium sulfate, and thismixture was gravity filtered. The resulting filtrate was concentrated,and the obtained solid was recrystallized from chloroform. Accordingly,6.4 g of a white powder was obtained in 40% yield, which was thesubstance to be produced.

Step 2: Synthesis of 2,8-diphenylbenzofuran

The synthesis scheme of Step 2 is shown in (E-2).

In a 300 mL three-neck flask were put 4.0 g (12 mmol) of2,8-dibromodibenzofuran, 3.0 g (24 mmol) of phenylboronic acid, and 0.55g (1.8 mmol) of tri(ortho-tolyl)phosphine. The air in the flask wasreplaced with nitrogen. To this mixture were added 45 mL of toluene, 15mL of ethanol, and 15 mL of an aqueous solution of potassium carbonate(2.0 mol/L). While the pressure was reduced, this mixture was stirred tobe degassed. Then, 81 mg (0.36 mmol) of palladium(II) acetate was addedto this mixture, and the mixture was stirred under a nitrogen stream at80° C. for 6 hours. After that, the aqueous layer of this mixture wasextracted with toluene, and the toluene solution and the organic layerwere combined and washed with saturated brine. The organic layer wasdried with magnesium sulfate. Then, this mixture was gravity filtered.The resulting filtrate was concentrated to give an oily substance, andthe oily substance was dissolved in about 20 mL of toluene. Thissolution was suction-filtered through Celite (manufactured by Wako PureChemical Industries, Ltd., Catalog No. 531-16855), alumina, and Florisil(manufactured by Wako Pure Chemical Industries, Ltd., Catalog No.540-00135). The resulting filtrate was concentrated to give an oilysubstance, and a mixed solvent of toluene and hexane was added to theoily substance. The mixture was irradiated with ultrasonic waves,whereby a solid was precipitated. This solid was collected by suctionfiltration to give 2.4 g of a white powder in 63% yield, which was thesubstance to be produced.

Step 3: Synthesis of 2,8-diphenyldibenzofuran-4-boronic acid

The synthesis scheme of Step 3 is shown in (E-3).

In a 200 mL three-neck flask was put 2.4 g (7.5 mmol) of2,8-diphenyldibenzofuran. The air in the flask was replaced withnitrogen. To this mixture was added 40 mL of tetrahydrofuran (THF), andthis solution was cooled to −80° C. Then, 5.6 mL (9.0 mmol) ofn-butyllithium (a 1.6 mol/L hexane solution) was dripped into thissolution with a syringe. After that, this solution was stirred for 2hours while its temperature was returned to room temperature. Then, thissolution was again cooled to −80° C., and 1.7 mL (15 mmol) of trimethylborate was added to this solution. This solution was stirred for 3 dayswhile its temperature was returned to room temperature. After that,about 30 mL of diluted hydrochloric acid (1.0 mol/L) was added to thissolution, followed by stirring for 1 hour. Then, the aqueous layer ofthis mixture was extracted with ethyl acetate, and the ethyl acetatesolution and the organic layer were combined and washed with saturatedbrine. The organic layer was dried with magnesium sulfate. Then, thismixture was gravity filtered. The resulting filtrate was concentrated togive an oily substance, and a mixed solvent of ethyl acetate and hexanewas added to the oily substance. The mixture was irradiated withultrasonic waves, whereby a solid was precipitated. This solid wascollected by suction filtration to give 2.2 g of a white powder in 82%□yield, which was the substance to be produced.

Step 4: Synthesis of 4-(3-bromophenyl)-2,8-diphenyldibenzofuran

The synthesis scheme of Step 4 is shown in (E-4).

In a 100 mL three-neck flask were put 1.7 g (6.0 mmol) of3-bromoiodobenzene and 2.2 g (6.0 mmol) of2,8-diphenyldibenzofuran-4-boronic acid. The air in the flask wasreplaced with nitrogen. To this mixture were added 30 mL of toluene and6.0 mL of an aqueous solution of sodium carbonate (2.0 mol/L). While thepressure was reduced, this mixture was stirred to be degassed. To thismixture was added 0.35 g (0.30 mmol) oftetrakis(triphenylphosphine)palladium(0), and the mixture was refluxedat 110° C. for 4 hours. After reaction, the aqueous layer was extractedwith ethyl acetate, and the ethyl acetate solution and the organic layerwere combined and washed with saturated brine. The organic layer wasdried with magnesium sulfate. Then, this mixture was gravity filtered.The resulting filtrate was concentrated to give an oily substance, andthe oily substance was dissolved in about 10 mL of toluene. Thissolution was suction-filtered through Celite (manufactured by Wako PureChemical Industries, Ltd., Catalog No. 531-16855), alumina, and Florisil(manufactured by Wako Pure Chemical Industries, Ltd., Catalog No.540-00135). The resulting filtrate was concentrated to give an oilysubstance, and hexane was added to the oily substance. The mixture wasirradiated with ultrasonic waves, whereby a solid was precipitated. Thissolid was collected by suction filtration to give 1.2 g of a whitepowder in 44% yield, which was the substance to be produced.

Step 5: Synthesis of 3-(2,8-diphenyldibenzofuran-4-yl)phenylboronic acid

The synthesis scheme of Step 5 is shown in (E-5).

In a 50 mL three-neck flask was put 1.2 g (2.5 mmol) of4-(3-bromophenyl)-2,8-diphenyldibenzofuran. The air in the flask wasreplaced with nitrogen. To this mixture was added 15 mL oftetrahydrofuran (THF), and this solution was cooled to −80° C. Then, 1.9mL (3.0 mmol) of n-butyllithium (a 1.6 mol/L hexane solution) wasdripped into this solution with a syringe. After that, this solution wasstirred at the same temperature for 1 hour. Then, 0.56 L (5.0 mmol) oftrimethyl borate was added to this solution, and the mixture was stirredfor 18 hours while its temperature was returned to room temperature.After that, about 10 mL of diluted hydrochloric acid (1.0 mol/L) wasadded to this solution, followed by stirring for 1 hour. Then, theaqueous layer of this mixture was extracted with ethyl acetate, and theethyl acetate solution and the organic layer were combined and washedwith saturated brine. The organic layer was dried with magnesiumsulfate. Then, this mixture was gravity filtered. The resulting filtratewas concentrated to give a solid, and a mixed solvent of chloroform andhexane was added to the solid. The mixture was irradiated withultrasonic waves, whereby a solid was precipitated. This solid wascollected by suction filtration to give 0.62 g of a light-brown powderin 58%□ yield, which was the substance to be produced.

Step 6: Synthesis of4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran(2mDBFPPA-III

The synthesis scheme of Step 6 is shown in (E-6).

In a 50 mL three-neck flask were put 0.62 g (1.3 mmol) of2-iode-9,10-diphenylanthracene, 0.60 g (1.3 mmol) of3-(2,8-diphenyldibenzofuran-4-yl)phenylboronic acid, and 99 mg (0.33mmol) of tri(ortho-tolyl)phosphine. The air in the flask was replacedwith nitrogen. To this mixture were added 10 mL of toluene, 3.0 mL ofethanol, and 2.0 mL of an aqueous solution of potassium carbonate (2.0mol/L). While the pressure was reduced, this mixture was stirred to bedegassed. To this mixture was added 15 mg (0.065 mmol) of palladium(II)acetate, and the mixture was stirred at 80° C. for 4 hours. Then, theaqueous layer of the obtained mixture was extracted with toluene, andthe toluene solution and the organic layer were combined and washed withsaturated brine. The organic layer was dried with magnesium sulfate.Then, this mixture was gravity filtered. The resulting filtrate wasconcentrated to given an oily substance, and the obtained oily substancewas purified by silica gel column chromatography to give a yellow oilysubstance. The chromatography was carried out using a mixed solventhaving a 3:1 ratio of hexane to toluene as a developing solvent.Recrystallization of the oily substance from a mixed solvent of tolueneand hexane gave 0.55 g of a yellow powder in 58% yield, which was thesubstance to be produced.

By a train sublimation method, 0.55 g of the obtained yellow powderedsolid was purified. In the purification, the yellow powdered solid washeated at 320° C. under a pressure of 3.0 Pa with a flow rate of argongas of 4.0 mL/min. After the purification, 0.50 g of a yellow solid wasobtained in a yield of 90%, which was the substance to be produced.

A nuclear magnetic resonance (NMR) method identified this compound as4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran(abbreviation: 2mDBFPPA-III), which was the substance to be produced.

¹H NMR data of the obtained compound are: ¹H NMR (CDCl₃, 300 MHz):δ=7.31-7.65 (m, 21H), 7.69-7.74 (m, 8H), 7.81-7.84 (m, 2H), 7.95 (dt,J₁=1.8 Hz, J₂=7.5 Hz, 1H), 8.03 (sd, J₁=1.5 Hz, 1H), 8.10 (s, 1H), 8.03(dd, J₁=1.5 Hz, J₂=12.3 Hz, 2H).

FIGS. 38A and 38B show the ¹H NMR charts. Note that FIG. 38B is a chartshowing an enlarged part of FIG. 38A in the range of 7.2 to 8.3 ppm.

Thermogravimetry-differential thermal analysis (TG-DTA) of 2mDBFPPA-III,which was obtained, was performed. A high vacuum differential typedifferential thermal balance (manufactured by Bruker AXS K.K., TG/DTA2410SA) was used for the measurement. The measurement was carried outunder a nitrogen stream (a flow rate of 200 mL/min) and a normalpressure at a temperature rising rate of 10° C./min. The relationshipbetween weight and temperature (thermogravimetry) demonstrates that thetemperature at which the weight at the start of the measurement isreduced by 5% (5% weight loss temperature) is 448° C., which isindicative of high heat resistance.

Further, FIG. 39A shows an absorption spectrum of a toluene solution of2mDBFPPA-III, and FIG. 39B shows an emission spectrum thereof. FIG. 40Ashows an absorption spectrum of a thin film of 2mDBFPPA-III, and FIG.40B shows an emission spectrum thereof. The absorption spectrum wasmeasured using an ultraviolet-visible spectrophotometer (V-550, producedby JASCO Corporation). The measurements were performed with samplesprepared in such a manner that the solution was put in a quartz cellwhile the thin film was obtained by evaporation onto a quartz substrate.The absorption spectrum of the solution was obtained by subtracting theabsorption spectra of quartz and toluene from those of quartz and thesolution, and the absorption spectrum of the thin film was obtained bysubtracting the absorption spectrum of a quartz substrate from those ofthe quartz substrate and the thin film. In FIGS. 39A and 39B and FIGS.40A and 40B, the horizontal axis represents wavelength (nm) and thevertical axis represents intensity (arbitrary unit). In the case of thetoluene solution, absorption was observed at around 365 nm, 385 nm and406 nm and the emission wavelengths were 423 nm and 447 nm (excitationwavelength: 385 nm). In the case of the thin film, absorption wasobserved at around 288 nm, 371 nm, 391 nm and 414 nm, and the emissionwavelengths were 439 nm and 459 nm (excitation wavelength: 413 nm).

The HOMO level and the LUMO level of the thin film of 2mDBFPPA-III weremeasured. The value of the HOMO level was obtained by conversion of avalue of the ionization potential measured with a photoelectronspectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in theatmosphere into a negative value. The value of the LUMO level wasobtained in such a manner that the absorption edge, which was obtainedfrom Tauc plot with an assumption of direct transition using data on theabsorption spectrum of the thin film of 2mDBFPPA-III which is shown inFIG. 40A, was regarded as an optical energy gap and added to the valueof the HOMO level. As a result, the HOMO level and LUMO level of2mDBFPPA-III were found to be −5.77 eV and −2.92 eV, respectively.

The oxidation characteristic and reduction characteristic of2mDBFPPA-III were measured. In the measurements of the oxidation andreduction characteristics, cyclic voltammetry (CV) measurement wasemployed, and an electrochemical analyzer (ALS model 600A, manufacturedby BAS Inc.) was used.

As a solution used in the CV measurement, dehydratedN,N-dimethylformamide (DMF, product of Sigma-Aldrich Inc., 99.8%,catalog No. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,catalog No. T0836), which was a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Further, the object to be measured wasdissolved in the solvent such that the concentration thereof was 1mmol/L. A platinum electrode (manufactured by BAS Inc., PTE platinumelectrode) was used as a working electrode, a platinum electrode(manufactured by BAS Inc., Pt counter electrode for VC-3, (5 cm)) wasused as an auxiliary electrode, and an Ag/Ag⁺ electrode (manufactured byBAS Inc., RE-5 reference electrode for nonaqueous solvent) was used as areference electrode. Note that the measurement was conducted at roomtemperature. In addition, the scan rate at the CV measurement was set to0.1 V/s in all the measurement.

The reduction characteristic of 2mDBFPPA-III was examined by 100measurement cycles in which the potential of the working electrode withrespect to the reference electrode was scanned from −1.59 V to −2.25 Vand then from −2.25 V to −1.59 V in each cycle. Similarly, the oxidationcharacteristic of 2mDBFPPA-III was evaluated by 100 measurement cyclesin which the potential of the working electrode with respect to thereference electrode was scanned from 0.25 V to 1.00 V and then from 1.00V to 0.25 V in each cycle.

According to the measurement results, a peak current corresponding tooxidation at around 0.88 V (vs. Ag/Ag⁺) and a peak current correspondingto reduction at around −2.16 V (vs. Ag/Ag⁺) were observed. FIG. 41 showsa graph of the results.

Even after as many as 100 scan cycles, 2mDBFPPA-III showed nosignificant change in the peak position of the CV curves representingoxidation and reduction and kept the peak intensity at 81% of theinitial intensity on the oxidation side and at 87% on the reductionside. Thus, it is understood that 2mDBFPPA-III is relatively stable,when subjected to repetitions of oxidation from a neutral state to anoxidized state and reduction from the oxidized state to the neutralstate or repetitions of reduction from a neutral state to a reducedstate and oxidation from the reduced state to the neutral state.

Example 8

In this example, a light-emitting element of one embodiment of thepresent invention will be described referring to FIG. 37A. Chemicalformulae of materials used in this example are shown below.

A method of fabricating Light-emitting Element 5 of this example willnow be described.

(Light-Emitting Element 5)

First, ITSO was deposited over the glass substrate 1100 by a sputteringmethod, whereby the first electrode 1101 was formed. Its thickness was110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode1101 is an electrode that functions as an anode of the light-emittingelement.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, a surface of the substrate was washed with water, bakedat 200° C. for one hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a substrate holder in a vacuumevaporation apparatus so that a surface of the substrate 1100 on whichthe first electrode 1101 was formed faced downward. The pressure in thevacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. Then, by anevaporation method using resistance heating,9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA) which is a substance having a high hole-transport property andmolybdenum(VI) oxide which is an acceptor substance were co-evaporatedto form the hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was 50 nm, and the weightratio of PCzPA to molybdenum(VI) oxide was controlled to be 4:2(=PCzPA:molybdenum(VI) oxide). Note that the co-evaporation methodrefers to an evaporation method in which evaporation is carried out froma plurality of evaporation sources at the same time in one treatmentchamber.

Next, PCzPA was deposited to a thickness of 10 nm over thehole-injection layer 1111, whereby the hole-transport layer 1112 wasformed.

Furthermore,4-[3-(9,10-diphenyl-2-anthryl)phenyl]-2,8-diphenyldibenzofuran(abbreviation: 2mDBFPPA-III) synthesized in Example 7 andN,N-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn) were co-evaporated to form the light-emittinglayer 1113 over the hole-transport layer. The weight ratio of2mDBFPPA-III to 1,6FLPAPrn was adjusted to 1:0.05(=2mDBFPPA-III:1,6FLPAPrn). The thickness of the light-emitting layer1113 was set to 30 nm.

Then, over the light-emitting layer 1113, a 10 nm thick layer of Alqand, a 15 nm thick layer of BPhen were deposited on the Alq layer,whereby the electron-transport layer 1114 including Alq and BPhen wasobtained.

Further, a 1 nm thick film of LiF was formed over the electron-transportlayer 1114 by evaporation, whereby the electron-injection layer 1115 wasformed.

Lastly, a 200 nm thick film of aluminum was formed by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 5 of this example was fabricated.

Note that, in the above evaporation process, evaporation was allperformed by a resistance heating method.

Table 9 shows element structures of Light-emitting Element 5 formed asdescribed above.

TABLE 9 Hole- Hole- Light- Electron- Electron- First injection transportemitting transport injection Second electrode layer layer layer layerlayer electrode Light-emitting ITSO PCzPA:MoOx PCzPA2mDBFPPA-III:1,6FLPAPrn Alq BPhen LiF Al element 5 110 nm (=4:2) 10 nm(=1:0.05) 10 nm 15 nm 1 nm 200 nm 50 nm 30 nm

Light-emitting Element 5 was sealed in a glove box containing a nitrogenatmosphere so as not to be exposed to air. Then, operationcharacteristics of the Light-emitting Element 5 were measured. Note thatthe measurement was carried out at room temperature (in the atmospherekept at 25° C.).

FIG. 42 shows luminance vs. current density characteristics ofLight-emitting Element 5. In FIG. 42, the horizontal axis representsluminance (cd/m²) and the vertical axis represents current efficiency(cd/A). FIG. 43 shows the voltage vs. luminance characteristics. In FIG.43, the horizontal axis represents applied voltage (V) and the verticalaxis represents luminance (cd/m²). FIG. 44 shows the luminance vs.external quantum efficiency characteristics. In FIG. 44, the horizontalaxis represents luminance (cd/m²) and the vertical axis representsexternal quantum efficiency (%). FIG. 45 shows the emission spectra witha current supply of 1 mA. In FIG. 45, the horizontal axis representswavelength (nm) and the vertical axis represents intensity (arbitraryunit). Further, Table 10 shows the voltage (V), current density(mA/cm²), CIE chromaticity coordinates (x, y), current efficiency(cd/A), and external quantum efficiency (%) of the light-emittingelement at a luminance of around 780 cd/m².

TABLE 10 External Current Current quantum Voltage density ChromaticityChromaticity Luminance efficiency efficiency (V) (mA/cm²) x y (cd/m²)(cd/A) (%) Light-emitting 4.2 11 0.15 0.21 780 7.4 5.1 element 5

As seen from FIG. 45 and the CIE chromaticity coordinates in Table 10,blue light emission is shown by Light-emitting Element 5, which wasformed. FIG. 42, FIG. 43, FIG. 44, and Table 10 reveal thatLight-emitting Element 5 exhibits good chromaticity, high currentefficiency, and high external quantum efficiency.

As described above, 2mDBFPPA-III produced in Example 7 was used as thehost material of the light-emitting layer, whereby the light-emittingelement achieved good chromaticity and high emission efficiency.

Next, Light-emitting Element 5 was subjected to reliability tests.Results of the reliability tests are shown in FIG. 46. In FIG. 46, thevertical axis represents normalized luminance (%) with an initialluminance of 100%, and the horizontal axis represents driving time (h)of the element. In the reliability tests, Light-emitting Element 5 ofthis example was driven under the conditions where the current densitywas constant and the initial luminance was 1000 cd/m². FIG. 46 showsthat Light-emitting Element 5 kept 81% of the initial luminance afterthe driving for 1300 hours. Thus, Light-emitting Element 5 shows highreliability. Furthermore, the results of the reliability testsdemonstrate that the light-emitting element to which one embodiment ofthe present invention is applied is effective in realizing alight-emitting element having a long lifetime.

Example 9 Synthesis Example 4

This example will show a method of synthesizing2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2mDBFPPA)) represented by Structural formula (158) described inEmbodiment 1.

Step 1: Synthesis of dibenzofuran-2-boronic acid

The synthesis scheme of Step 1 is shown in (F-1).

In a 300 mL three-neck flask was put 3.6 g (14 mmol) of3-bromodibenzofuran. The air in the flask was replaced with nitrogen. Tothis mixture was added 70 mL of THF, and this solution was cooled to−80° C. Then, 10 mL (16 mmol) of n-butyllithium (a 1.6 mol/L hexanesolution) was dripped into this solution with a syringe. After that,this solution was stirred at the same temperature for 2 hours. Then, 3.4mL (30 mmol) of trimethyl borate was added to this solution, and themixture was stirred for 4 days while its temperature was returned toroom temperature. After that, about 30 mL of diluted hydrochloric acid(1.0 mol/L) was added to this solution, followed by stirring for 1 hour.Then, the aqueous layer of this mixture was extracted with ethylacetate, and the ethyl acetate solution and the organic layer werecombined and washed with saturated brine. The organic layer was driedwith magnesium sulfate. Then, this mixture was gravity filtered. Theresulting filtrate was concentrated to give a solid. The solid waswashed with hexane, whereby 0.70 g of a white powder was obtained in 27%yield, which was the substance to be produced.

Step 2: Synthesis of 2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran(Abbreviation: 2mDBFPPA

The synthesis scheme of Step 2 is shown in (F-2).

In a 50 mL three-neck flask were put 1.2 g (2.4 mmol) of2-(3-bromophenyl)-9,10-diphenylanthracene, 0.52 g (2.4 mmol) ofdibenzofuran-2-boronic acid, and 0.18 g (0.60 mmol) oftri(ortho-tolyl)phosphine. The air in the flask was replaced withnitrogen. To this mixture were added 10 mL of toluene, 3.0 mL ofethanol, and 3.0 mL of an aqueous solution of sodium carbonate (2.0mol/L). While the pressure was reduced, this mixture was stirred to bedegassed. To this mixture was added 27 mg (0.12 mmol) of palladium(II)acetate, and the mixture was stirred at 80° C. for 3 hours. Then, theaqueous layer of the obtained mixture was extracted with toluene, andthe toluene solution and the organic layer were combined and washed withsaturated brine. The organic layer was dried with magnesium sulfate.Then, this mixture was gravity filtered. The resulting filtrate wasconcentrated, and the obtained oily substance was purified by silica gelcolumn chromatography to give a yellow oily substance. Thechromatography was carried out using a mixed solvent having a 5:1 ratioof hexane to toluene as a developing solvent, whereby an oily substancewas obtained. Recrystallization of the oily substance from a mixedsolvent of toluene and hexane gave 0.40 g of a yellow powder in 29%yield, which was the substance to be produced.

By a train sublimation method, 0.40 g of the obtained yellow powderedsolid was purified. In the purification, the yellow powdered solid washeated at 270° C. under a pressure of 2.6 Pa with a flow rate of argongas of 5.0 mL/min. After the purification, 0.35 g of a yellow solid wasobtained in a yield of 87%, which was the substance to be produced.

A nuclear magnetic resonance (NMR) method identified this compound as2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2mDBFPPA), which was the substance to be produced.

¹H NMR data of the obtained compound are: ¹H NMR (CDCl₃, 300 MHz):δ=7.32-7.38 (m, 3H), 7.43-7.74 (m, 20H), 7.81-7.83 (m, 2H), 7.98-8.01(m, 2H), 8.13 (sd, J₁=1.8 Hz, 1H).

FIGS. 47A and 47B show the ¹H NMR charts. Note that FIG. 47B is a chartshowing an enlarged part of FIG. 47A in the range of 7.2 to 8.2 ppm.

Thermogravimetry-differential thermal analysis (TG-DTA) of 2mDBFPPA,which was obtained, was performed. A high vacuum differential typedifferential thermal balance (manufactured by Bruker AXS K.K., TG/DTA2410SA) was used for the measurement. The measurement was carried outunder a nitrogen stream (a flow rate of 200 mL/min) and a normalpressure at a temperature rising rate of 10° C./min. The relationshipbetween weight and temperature (thermogravimetry) demonstrates that thetemperature at which the weight at the start of the measurement isreduced by 5% (5% weight loss temperature) is 415° C., which isindicative of high heat resistance.

Further, FIG. 48A shows an absorption spectrum of a toluene solution of2mDBFPPA, and FIG. 48B shows an emission spectrum thereof. FIG. 49Ashows an absorption spectrum of a thin film of 2mDBFPPA, and FIG. 49Bshows an emission spectrum thereof. The absorption spectrum was measuredusing an ultraviolet-visible spectrophotometer (V-550, produced by JASCOCorporation). The measurements were performed with samples prepared insuch a manner that the solution was put in a quartz cell while the thinfilm was obtained by evaporation onto a quartz substrate. The absorptionspectrum of the solution was obtained by subtracting the absorptionspectra of quartz and toluene from those of quartz and the solution, andthe absorption spectrum of the thin film was obtained by subtracting theabsorption spectrum of a quartz substrate from those of the quartzsubstrate and the thin film. In FIGS. 48A and 48B and FIGS. 49A and 49B,the horizontal axis represents wavelength (nm) and the vertical axisrepresents intensity (arbitrary unit). In the case of the toluenesolution, absorption was observed at around 291 nm, 366 nm, 384 nm, and406 nm and the emission wavelengths were 423 nm and 446 nm (excitationwavelength: 385 nm). In the case of the thin film, absorption wasobserved at around 246 nm, 293 nm, 371 nm and 413 nm, and the emissionwavelengths were 437 nm and 459 nm (excitation wavelength: 413 nm).

The HOMO level and the LUMO level of the thin film of 2mDBFPPA weremeasured. The value of the HOMO level was obtained by conversion of avalue of the ionization potential measured with a photoelectronspectrometer (AC-2, manufactured by Riken Keiki Co., Ltd.) in theatmosphere into a negative value. The value of the LUMO level wasobtained in such a manner that the absorption edge, which was obtainedfrom Tauc plot with an assumption of direct transition using data on theabsorption spectrum of the thin film of 2mDBFPPA which is shown in FIG.49A, was regarded as an optical energy gap and added to the value of theHOMO level. As a result, the HOMO level and LUMO level of 2mDBFPPA werefound to be −5.71 eV and −2.85 eV, respectively.

The oxidation characteristic and reduction characteristic of 2mDBFPPAwere measured. In the measurements of the oxidation and reductioncharacteristics, cyclic voltammetry (CV) measurement was employed, andan electrochemical analyzer (ALS model 600A, manufactured by BAS Inc.)was used.

As a solution used in the CV measurement, dehydratedN,N-dimethylformamide (DMF, product of Sigma-Aldrich Inc., 99.8%,catalog No. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,catalog No. T0836), which was a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Further, the object to be measured wasdissolved in the solvent such that the concentration thereof was 1mmol/L. A platinum electrode (manufactured by BAS Inc., PTE platinumelectrode) was used as a working electrode, a platinum electrode(manufactured by BAS Inc., Pt counter electrode for VC-3, (5 cm)) wasused as an auxiliary electrode, and an Ag/Ag⁺ electrode (manufactured byBAS Inc., RE-5 reference electrode for nonaqueous solvent) was used as areference electrode. Note that the measurement was conducted at roomtemperature. In addition, the scan rate at the CV measurement was set to0.1 V/s in all the measurement.

The reduction characteristic of 2mDBFPPA was examined by 100 measurementcycles in which the potential of the working electrode with respect tothe reference electrode was scanned from −1.56 V to −2.27 V and thenfrom −2.27 V to −1.56 V in each cycle. Similarly, the oxidationcharacteristic of 2mDBFPPA was evaluated by 100 measurement cycles inwhich the potential of the working electrode with respect to thereference electrode was scanned from 0.20 V to 1.05 V and then from 1.05V to 0.20 V in each cycle.

According to the measurement results, a peak current corresponding tooxidation at around 0.95 V (vs. Ag/Ag⁺) and a peak current correspondingto reduction at around −2.22 V (vs. Ag/Ag⁺) were observed. FIG. 50 showsa graph of the results.

Even after as many as 100 scan cycles, 2mDBFPPA showed no significantchange in the peak position of the CV curves representing oxidation andreduction and kept the peak intensity at 73% of the initial intensity onthe oxidation side and at 89% on the reduction side. Thus, it isunderstood that 2mDBFPPA is relatively stable, when subjected torepetitions of oxidation from a neutral state to an oxidized state andreduction from the oxidized state to the neutral state or repetitions ofreduction from a neutral state to a reduced state and oxidation from thereduced state to the neutral state.

Example 10

In this example, a light-emitting element of one embodiment of thepresent invention will be described referring to FIG. 37A. Chemicalformulae of materials used in this example are shown below.

A method of fabricating Light-emitting Element 6 of this example willnow be described.

(Light-Emitting Element 6)

First, ITSO was deposited over the glass substrate 1100 by a sputteringmethod, whereby the first electrode 1101 was formed. Its thickness was110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode1101 is an electrode that functions as an anode of the light-emittingelement.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, a surface of the substrate was washed with water, bakedat 200° C. for one hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a substrate holder in a vacuumevaporation apparatus so that a surface of the substrate 1100 on whichthe first electrode 1101 was formed faced downward. The pressure in thevacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. Then, by anevaporation method using resistance heating, PCzPA which is a substancehaving a high hole-transport property and molybdenum(VI) oxide which isan acceptor substance were co-evaporated to form the hole-injectionlayer 1111 over the first electrode 1101. The thickness of thehole-injection layer 1111 was 50 nm, and the weight ratio of PCzPA tomolybdenum(VI) oxide was controlled to be 4:2 (=PCzPA:molybdenum(VI)oxide). Note that the co-evaporation method refers to an evaporationmethod in which evaporation is carried out from a plurality ofevaporation sources at the same time in one treatment chamber.

Next, PCzPA was deposited to a thickness of 10 nm over thehole-injection layer 1111, whereby the hole-transport layer 1112 wasformed.

Furthermore, 2-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran(abbreviation: 2mDBFPPA) synthesized in Example 9 and 1,6FLPAPrn wereco-evaporated to form the light-emitting layer 1113 over thehole-transport layer. The weight ratio of 2mDBFPPA to 1,6FLPAPrn wasadjusted to 1:0.05 (=2mDBFPPA:1,6FLPAPrn). The thickness of thelight-emitting layer 1113 was set to 30 nm.

Then, over the light-emitting layer 1113, a 10 nm thick layer of Alqand, a 15 nm thick layer of BPhen were deposited on the Alq layer,whereby the electron-transport layer 1114 including Alq and BPhen wasobtained.

Further, a 1 nm thick film of LiF was formed over the electron-transportlayer 1114 by evaporation, whereby the electron-injection layer 1115 wasformed.

Lastly, a 200 nm thick film of aluminum was formed by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 6 of this example was fabricated.

Note that, in the above evaporation process, evaporation was allperformed by a resistance heating method.

Table 11 shows element structures of Light-emitting Element 6 formed asdescribed above.

TABLE 11 Hole- Hole- Light- Electron- Electron- First injectiontransport emitting transport injection Second electrode layer layerlayer layer layer electrode Light-emitting ITSO PCzPA:MoOx PCzPA2mDBFPPA:1,6FLPAPrn Alq BPhen LiF Al element 6 110 nm (=4:2) 10 nm(=1:0.05) 10 nm 15 nm 1 nm 200 nm 50 nm 30 nm

Light-emitting Element 6 was sealed in a glove box containing a nitrogenatmosphere so as not to be exposed to air. Then, operationcharacteristics of the Light-emitting Element 6 were measured. Note thatthe measurement was carried out at room temperature (in the atmospherekept at 25° C.).

FIG. 51 shows luminance vs. current density characteristics ofLight-emitting Element 6. In FIG. 51, the horizontal axis representsluminance (cd/m²) and the vertical axis represents current efficiency(cd/A). FIG. 52 shows the voltage vs. luminance characteristics. In FIG.52, the horizontal axis represents applied voltage (V) and the verticalaxis represents luminance (cd/m²). FIG. 53 shows the luminance vs.external quantum efficiency characteristics. In FIG. 53, the horizontalaxis represents luminance (cd/m²) and the vertical axis representsexternal quantum efficiency (%). FIG. 54 shows the emission spectra witha current supply of 1 mA. In FIG. 54, the horizontal axis representswavelength (nm) and the vertical axis represents intensity (arbitraryunit). Further, Table 12 shows the voltage (V), current density(mA/cm²), CIE chromaticity coordinates (x, y), current efficiency(cd/A), and external quantum efficiency (%) of the Light-emittingElement 6 at a luminance of around 1200 cd/m².

TABLE 12 External Current Current quantum Voltage density ChromaticityChromaticity Luminance efficiency efficiency (V) (mA/cm²) x y (cd/m²)(cd/A) (%) Light-emitting 4.4 16 0.16 0.26 1200 7.8 4.6 element 6

As seen from FIG. 54 and the CIE chromaticity coordinates in Table 12,blue light emission is shown by Light-emitting Element 6, which wasformed. FIG. 51, FIG. 52, FIG. 53, and Table 12 reveal thatLight-emitting Element 6 exhibits good chromaticity, high currentefficiency, and high external quantum efficiency.

As described above, 2mDBFPPA produced in Example 9 was used as the hostmaterial of the light-emitting layer, whereby the light-emitting elementachieved good chromaticity and high emission efficiency.

Reference Example 1

A method for synthesizing4-phenyl-4′-[4-(9-phenylfluoren-9-yl)phenyl]triphenylamine(abbreviation: BPAFLBi) used in Examples 3 and 4 above will bespecifically described. The structure of BPAFLBi is shown below.

Step 1: Synthesis Method of 9-(4′-bromo-4-biphenyl)-9-phenylfluorene

In a 500-mL three-neck flask was put 5.1 g (22 mmol) of 2-bromobiphenyl.The air in the flask was replaced with nitrogen. Then, 200 mL oftetrahydrofuran (abbreviation: THF) was added to the mixture, and themixture was cooled to −78° C. Then, 14 mL (22 mmol) of an n-butyllithiumhexane solution was dripped into this mixture solution, and the mixturewas stirred for 2.5 hours. After that, 6.7 g (20 mmol) of9-(4′-bromobiphenylyl)-9-phenylfluoren was added to this mixture, andthe mixture was stirred at −78° C. for 2 hours and then at roomtemperature for 85 hours.

After reaction, 1N-diluted hydrochloric acid was added to this reactionsolution until the mixed solution was made acid, and the mixture wasstirred for 4 hours. The mixture was washed with water. After that,magnesium sulfate was added to the mixture so that moisture is removed.This suspension was filtered, and the filtrate was concentrated. Theresulting substance was purified by silica gel column chromatography(the developing solvent was hexane). The obtained fractions wereconcentrated, followed by addition of methanol thereto. The resultingsubstance was irradiated with ultrasonic waves, and then recrystallizedto give a white powder, which was the substance to be produced.

In a 200-mL recovery flask were put this white powder, 50 mL of glacialacetic acid, and 1.0 mL of hydrochloric acid. The mixture was heated,and stirred under a nitrogen atmosphere at 130° C. for 2.5 hours to bereacted.

After reaction, this reaction mixture solution was filtered. Theresulting filtrate was dissolved in 100 mL of toluene, and the mixturewas washed with water, aqueous sodium hydroxide, and water in thisorder. Magnesium sulfate was added to the mixture so that moisture isremoved. This suspension was filtered, and the resulting filtrate wasconcentrated. Acetone and methanol were added to the resultingsubstance. The mixture was irradiated with ultrasonic waves and thenrecrystallized to give 6.3 g of a white powder in a yield of 67%, whichwas the substance to be produced. The reaction scheme is shown in thefollowing (J-1).

Step 2: Synthesis Method of4-phenyl-4′-[4-(9-phenylfluoren-9-yl)phenyl]triphenylamine(Abbreviation: BPAFLBi]

In a 100 mL three-neck flask were put 3.8 g (8.0 mmol) of9-(4′-bromo-4-biphenyl)-9-phenylfluorene, 2.0 g (8.0 mmol) of4-phenyl-diphenylamine, 1.0 g (10 mmol) of sodium tert-butoxide, and 23mg (0.04 mmol) of bis(dibenzylideneacetone)palladium(0). The air in theflask was replaced with nitrogen. Then, 20 mL of dehydrated xylene wasadded to this mixture. After the mixture was degassed while beingstirred under reduced pressure, 0.2 mL (0.1 mmol) oftri(tert-butyl)phosphine (10 wt % hexane solution) was added to themixture. This mixture was heated and stirred under a nitrogen atmosphereat 110° C. for 2 hours to be reacted.

After reaction, 200 mL of toluene was added to the reaction mixturesolution, and the resulting suspension was filtered through Florisil(produced by Wako Pure Chemical Industries, Ltd., Catalog No. 540-00135)and Celite (produced by Wako Pure Chemical Industries, Ltd., Catalog No.531-16855). The resulting filtrate was concentrated. The resultingsubstance was purified by silica gel column chromatography (thedeveloping solvent has a 1:4 ratio of toluene to hexane). The obtainedfractions were concentrated, and acetone and methanol were added to themixture. The mixture was irradiated with ultrasonic waves and thenrecrystallized to give 4.4 g of a white powder in a yield of 86%, whichwas the substance to be produced. The reaction scheme of the abovesynthesis method is shown in the following (J-2).

The Rf values of the produced substance,9-(4′-bromo-4-biphenyl)-9-phenylfluorene, and 4-phenyl-diphenylaminewere respectively 0.51, 0.56, and 0.28, which were found by silica gelthin layer chromatography (TLC) (the developing solvent has a 1:10 ratioof ethyl acetate to hexane).

The compound obtained through the above Step 2 was subjected to anuclear magnetic resonance (NMR) method. The measurement data are shownbelow. The measurement results indicate that the obtained compound wasBPAFLBi, which is a fluorene derivative.

¹H NMR (CDCl₃, 300 MHz): δ (ppm)=7.04 (t, J=6.6, 1H), 7.12-7.49 (m,30H), 7.55-7.58 (m, 2H), 7.77 (d, J=7.8, 2H).

Reference Example 2

A method for synthesizing4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)used in Examples 5 and 6 above will be specifically described. Thestructure of BPAFLP is shown below.

Step 1: Synthesis Method of 9-(4-bromophenyl)-9-phenylfluorene

In a 100-mL three-neck flask, 1.2 g (50 mmol) of magnesium was heatedand stirred under reduced pressure for 30 minutes to be activated. Afterthe flask was cooled to room temperature and was made to have a nitrogenatmosphere, several drops of dibromoethane were added, so that foamformation and heat generation were confirmed. After 12 g (50 mmol) of2-bromobiphenyl dissolved in 10 mL of diethyl ether was slowly drippedinto this mixture, the mixture was stirred and heated under reflux for2.5 hours. Accordingly, a Grignard reagent was prepared.

In a 500-mL three-neck flask were put 10 g (40 mmol) of4-bromobenzophenone and 100 mL of diethyl ether. After the Grignardreagent prepared as above was slowly dripped into this mixture, themixture was heated and stirred under reflux for 9 hours.

After reaction, this mixture was filtered to give a residue. The residuewas dissolved in 150 mL of ethyl acetate, and 1N-hydrochloric acid wasadded to the mixture, which was then stirred for 2 hours until it wasmade acid. The organic layer of the liquid was washed with water. Then,magnesium sulfate was added thereto so that moisture is removed. Thissuspension was filtered, and the resulting filtrate was concentrated togive a candy-like substance.

In a 500-mL recovery flask were put this candy-like substance, 50 mL ofglacial acetic acid, and 1.0 mL of hydrochloric acid. The mixture washeated and stirred under a nitrogen atmosphere at 130° C. for 1.5 hoursto be reacted.

After reaction, this reaction mixture solution was filtered to give aresidue. The residue was washed with water, aqueous sodium hydroxide,water, and methanol in this order. Then, the mixture was dried to give11 g of a white powder in 69% yield, which was the substance to beproduced. The reaction scheme of the synthesis method is shown in thefollowing (J-3).

Step 2: Synthesis Method of4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (Abbreviation: BPAFLP

In a 100-mL three-neck flask were put 3.2 g (8.0 mmol) of9-(4-bromophenyl)-9-phenylfluorene, 2.0 g (8.0 mmol) of4-phenyl-diphenylamine, 1.0 g (10 mmol) of sodium tert-butoxide and 23mg (0.04 mmol) of bis(dibenzylideneacetone)palladium(0). The air in theflask was replaced with nitrogen. Then, 20 mL of dehydrated xylene wasadded to this mixture. After the mixture was degassed while beingstirred under reduced pressure, 0.2 mL (0.1 mmol) oftri(tert-butyl)phosphine (a 10 wt % hexane solution) was added to themixture. This mixture was heated and stirred under a nitrogen atmosphereat 110° C. for 2 hours to be reacted.

After reaction, 200 mL of toluene was added to the reaction mixturesolution, and the resulting suspension was filtered through Florisil(produced by Wako Pure Chemical Industries, Ltd., Catalog No. 540-00135)and Celite (produced by Wako Pure Chemical Industries, Ltd., Catalog No.531-16855). The filtrate was concentrated, and the resulting substancewas purified by silica gel column chromatography (the developing solventhas a 1:4 ratio of toluene to hexane). The obtained fractions wereconcentrated, and acetone and methanol were added to the mixture. Themixture was irradiated with ultrasonic waves and then recrystallized togive 4.1 g of a white powder in 92% yield, which was the substance to beproduced. The reaction scheme of the above synthesis method is shown inthe following (J-4).

The Rf values of the produced substance,9-(4-bromophenyl)-9-phenylfluorene, and 4-phenyl-diphenylamine wererespectively 0.41, 0.51, and 0.27, which were found by silica gel thinlayer chromatography (TLC) (the developing solvent has a 1:10 ratio ofethyl acetate to hexane).

The compound obtained through the above Step 2 was subjected to anuclear magnetic resonance (NMR) method. The measurement data are shownbelow. The measurement results indicate that the obtained compound wasBPAFLP, which is a fluorene derivative.

¹H NMR (CDCl₃, 300 MHz): δ (ppm)=6.63-7.02 (m, 3H), 7.06-7.11 (m, 6H),7.19-7.45 (m, 18H), 7.53-7.55 (m, 2H), 7.75 (d, J=6.9, 2H).

Reference Example 3

In this example,N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn) used in Examples 8 and 10 was produced.

Step 1: Synthesis Method of 4-(9-phenyl-9H-fluoren-9-yl)diphenylamine(Abbreviation: FLPA

In a 200 mL three-neck flask were put 5.8 g (14.6 mmol) of9-(4-bromophenyl)-9-phenylfluorene, 1.7 mL (18.6 mmol) of aniline, and4.2 g (44.0 mmol) of sodium tert-butoxide. The air in the flask wasreplaced with nitrogen. To this mixture were added 147.0 mL of tolueneand 0.4 mL of a 10 wt % hexane solution of tri(tert-butyl)phosphine. Thetemperature of this mixture was set to 60° C., and 66.1 mg (0.1 mmol) ofbis(dibenzylideneacetone)palladium(0) was added to the mixture, followedby stirring for 3.5 hours. After the stirring, the mixture wassuction-filtered through Florisil (produced by Wako Pure ChemicalIndustries, Ltd., Catalog No. 540-00135), Celite (produced by Wako PureChemical Industries, Ltd., Catalog No. 531-16855), and alumina. Theresulting filtrate was concentrated to give a solid, which was thenpurified by silica gel column chromatography (the developing solvent hasa 2:1 ratio of hexane to toluene). The obtained fractions wereconcentrated to give 6.0 g of a while solid in 99% yield, which was thesubstance to be produced. The synthesis scheme of Step 1 is shown in(E1-2) below.

Step 2: Synthesis Method of4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-fluoren-9-yl)triphenylamine(Abbreviation: FLPAPA

In a 50 mL three-neck flask were put 0.4 g (1.2 mmol) of1,6-dibromopyrene, 1.0 g (2.4 mmol) of4-(9-phenyl-9H-fluoren-9-yl)diphenylamine (abbreviation: FLPA) and 0.3 g(3.6 mmol) of sodium tert-butoxide. The air in the flask was replacedwith nitrogen. To this mixture were added 11.5 mL of toluene and 0.2 mLof a 10 wt % hexane solution of tri(tert-butyl)phosphine. Thetemperature of this mixture was set to 70° C., and 31.1 mg (0.05 mmol)of bis(dibenzylideneacetone)palladium(0) was added to the mixture,followed by stirring for 4.0 hours. After the stirring, the mixture wassuction-filtered through Florisil, Celite, and alumina. The resultingfiltrate was concentrated to give a solid, which was then purified bysilica gel column chromatography (the developing solvent waschloroform). The obtained fractions were concentrated to give a yellowsolid. The obtained solid was washed with a mixed solvent of toluene andhexane, and then the mixture was suction-filtered to give a yellowsolid. The obtained yellow solid was washed with a mixed solvent ofchloroform and hexane, whereby 0.8 g of a pale yellow powdered solid wasobtained in 68% yield, which was the substance to be produced.

By a train sublimation method, 0.8 g of the obtained yellow solid waspurified. Under a pressure of 2.7 Pa with a flow rate of argon at 5mL/min, the sublimation purification was carried out at 360° C. Afterthe purification, 0.4 g of the substance to be produced was obtained ina yield of 56%. The synthesis scheme of the above step is shown in thefollowing (E2).

A nuclear magnetic resonance (NMR) method and a mass spectrometryidentified this compound asN,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn), which was the substance to be produced.

¹H NMR data of the obtained compound are: ¹H NMR (CDCl₃, 300 MHz):δ=6.88-6.91 (m, 6H), 7.00-7.03 (m, 8H), 7.13-7.40 (m, 26H), 7.73-7.80(m, 6H), 7.87 (d, J=9.0 Hz, 2H), 8.06-8.09 (m, 4H).

This application is based on Japanese Patent Application serial no.2009-260240 filed with the Japan Patent Office on Nov. 13, 2009, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting device comprising: a firstelectrode over a substrate; a light-emitting layer comprising adibenzofuran derivative over the first electrode; and a second electrodeover the light-emitting layer, wherein the dibenzofuran derivativecomprises an anthracene skeleton and a phenylene group, and wherein2-position of the anthracene skeleton and 4-position of a dibenzofuranskeleton are bonded through the phenylene group at meta-position of thephenylene group.
 2. The light-emitting device according to claim 1,wherein the light-emitting layer comprises the dibenzofuran derivativeand a light-emitting substance.
 3. The light-emitting device accordingto claim 1, wherein the dibenzofuran derivative comprises a structurerepresented by General Formula (G1),

wherein A represents a substituent represented by General Formula (S1)or (S2), wherein Ar¹ and Ar² independently represent a substituted orunsubstituted aryl group having 6 to 13 carbon atoms, wherein R¹ to R¹⁸independently represent any of hydrogen, an alkyl group having 1 to 4carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms, and wherein X represents oxygen.
 4. A light-emittingdevice comprising: a first electrode over a substrate; a hole-injectionlayer over the first electrode; a light-emitting layer comprising adibenzofuran derivative over the hole-injection layer; and a secondelectrode over the light-emitting layer, wherein the hole-injectionlayer comprises a first organic compound and an electron acceptor,wherein the dibenzofuran derivative comprises an anthracene skeleton anda phenylene group, and wherein 2-position of the anthracene skeleton and4-position of a dibenzofuran skeleton are bonded through the phenylenegroup at meta-position of the phenylene group.
 5. The light-emittingdevice according to claim 4, wherein the light-emitting layer comprisesthe dibenzofuran derivative and a light-emitting substance.
 6. Thelight-emitting device according to claim 4, wherein the dibenzofuranderivative comprises a structure represented by General Formula (G1),

wherein A represents a substituent represented by General Formula (S1)or (S2), wherein Ar¹ and Ar² independently represent a substituted orunsubstituted aryl group having 6 to 13 carbon atoms, wherein R¹ to R¹⁸independently represent any of hydrogen, an alkyl group having 1 to 4carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms, and wherein X represents oxygen.
 7. The light-emittingdevice according to claim 4, wherein the electron acceptor is a secondorganic compound.
 8. A light-emitting device comprising: a firstelectrode over a substrate; a first layer over the first electrode; alight-emitting layer comprising a dibenzofuran derivative and alight-emitting substance over the first layer; a second layer over thelight-emitting layer; and a second electrode over the second layer,wherein the second layer comprises a metal complex having a quinolineskeleton, wherein the dibenzofuran derivative comprises an anthraceneskeleton, wherein each of the benzene rings of a dibenzofuran skeletonin the dibenzofuran derivative is substituted, wherein the anthraceneskeleton is substituted with an aryl group having 6 to 13 carbon atomsat a 9-or 10-position, wherein the light-emitting substance comprises afluoren skeleton bonded to an amine skeleton, and wherein the firstlayer comprises a carbazole derivative.
 9. The light-emitting deviceaccording to claim 8, wherein the first electrode is an anode comprisingan indium-tin-oxide.
 10. The light-emitting device according to claim 8,wherein two substituents of one of the benzene rings form a ring.